I had to disinfect the water well the other week and after the 24hrs of disinfecting with bleach (3litres for a 100ft well), I could not pump the well water to waste as we have a river nearby so I had to turn on the taps and empty it into the septic tank.
I know you're not supposed to put bleach in but others suggested that it would settle after a week and restart.
The sump pump now seems to smell of sewer when we come back. The sump pump only pumps water from the basement floor to the tank.
Is it possible that the bleach has just temporarily reduced bacteria so the backteria are not yet doing their job properly in the tank?
I thought maybe we could have overloaded it with water (I did leave the taps on for about 2 hrs) but I would have thought if it overfilled then it would have gone down the septic drain pipes?
Anything I can do to check what the issue before calling the guys to clean out the tank (it was last emptied 2 years ago but it hasn't been used much in that time, maybe once every 2 weeks)?

 

All of that chlorine in your septic system couldn't have been very healthy for the colony of beneficial bacteria that you want to reside there. You may have to replenish the bacteria with a product like Rid-X or something else that performs similarly. With normal usage, it ought to come back anyway; but it may take a while longer.

There must not be a trap (fumes) on the drain from the sump pump to the septic tank ?

 

Is it possible to open the pump?
Also, how can you ever tell whether the septic tank is near full?

 

You have a lot of information, but the terms you used don't quite make sense.

Just addressing the problem you asked about:
it sounds like the septic tank and drain field were flooded with too much water, so the raw sewage from the septic system siphoned back into your sump pump and under the house.

 

When I say a sump pump, it's not pumping water from the basement, it is dry in there as the "basement" is actually a liveable area. However, it is below the level of the septic tank so I guess it's an ejector pump? The upstairs of the house drains to the septic tank by gravity but if youflush the toilet downstairs or run the tap, it pumps. I added some pictures...I also answered some of the questions above...
The amount of bleach seems to differ...I saw many sites saying at least 2-3litres per 100ft well.

Sixth - fixing this.
(A) the well - it may have way too much chlorine in it.
Take off the cap, do you still smell chlorine?
You may want to siphon the chlorinated water out of the well with a garden hose (assuming you have enough elevation change) so the remaining chlorine doesn't impact the pump seals.
There's nothing in it anymore, the chlorine was cleared out a week ago by running the taps, hence all the chlorine in the septic tank.

(B) the water system - putting lots of chlorine through the system may cause or loosen the scale or muck, keep an eye out for clogged aerators or shower heads.

(C) the sump pump - I'd pickup a replacement pump and one-way valve next time you are out,
don't know what the chlorine did to the pump.
Can I just undo the bolts and replace the valve?

(D) the septic system - after dumping all that water into the system, you may see blowouts in the yard or sand mound. May also need to dig up the D-box to check whether the surge of chlorinated water dislodged scum and/or blocked the distribution box.
There is nothing blowing out in the yard, no mounds, nothing flowing back up the pipes to the sink, no gurgling toilets.
How do you tell when the septic tank is full?

(E) the sump pit - sounds like you have had raw sewage siphoning back into the house from the septic system. Eh, yuck.
You'll want to disconnect the sump pump from the septic system, then run the pump to remove the water/raw sewage that is in the pit and/or seepage area under the house.
Possible, but wouldn't flushing it just get rid of it eventually?

Not really sure about a good way to deal with / disinfect raw sewage that has gotten into the sump and under the house. How would it have gotten under the house? The pump is sealed.
My best guess is that you'll need to temporarily disconnect the sump pump float,

 

Hi qwerty –

I’m not a pro or expert but if you look at the bottom of page on the link below, you will see a diagram of a septic tank and field. The “liquid” in the figure (between the scum and sludge) is the effluent that goes out to the field. The normal operating level in the tank is constant; it stays at the level of that output “sanitary tee”.

Septic Field Problems: Understanding the Biomat | SepticHelp.org

But what changes over time is the thickness of the scum level and the sludge level, decreasing therefore the thickness of the liquid level. The idea is that you don’t let the scum and/or sludge levels get too thick or the stuff going out the sanitary tee will have solids in it clogging up the drain field. Thus the tank needs to be pumped out periodically.

Here is a link that tells how you can measure these things yourself if you wanted to. (I made the poles described and made the measurements, pretty easy to do.)

Inspecting Your Septic Tank

But it seems to me that if you had your tank pumped 2 years ago and you only use the system about every 2 weeks, then I would think no way do you need your tank pumped. But I don’t know whether pumping would have any effect on a bacteria problem if one exists, or the odor problem you are experiencing. I really don’t know that much about it.

But good luck!