There are so many conditions and combination of conditions that can cause this it's tough to pinpoint with so little info & a few pix.

and grew a lot of chillies before stopping flowering before winter.

So can we assume they're indoors? If yes how much sunlight or artificial light are they getting? What's the temperature?

After re-potting them into a bigger pot with soil considered 'more suitable' for this plant

Always use "soil" for the stuff in an outdoor ground garden and "potting media" for what we put in pots. So what did you use? Transplant shock is a possibility but usually pepper plants overcome that.

At first i thought I might have over watered it therefore I put it in a warm place and didn't water it for a while but the problem carries on.

Over watering is often an issue and it could take awhile for the plant to recover. The growing media in your pix looks very wet to me?

@the_northeast_chileman,
Thank you for your reply!

'So can we assume they're indoors? If yes how much sunlight or artificial light are they getting? What's the temperature?'

Yes, the chillies have always been kept indoors and they have always been kept at a room temperature, most of the times on a desk above a PC which runs hot to give it more warmth which for the past months seemed to cause no issues, during the night it does get cold therefore on the worst nights I place it on a heating pad (not a temperature too high, just slightly warm). The only light it receives is the scarce daylight, which in the UK is limited around this time.

'So what did you use?'

The soil it has now is the usual 'garden soil' which can be bought in bags with a small amount of compost added to it. It was re-potted to this as the previous soil was simply taken from my back garden and had a very clay-like quality which made it retain the water for too long.

'The growing media in your pix looks very wet to me?'

Yes I agree it does look very wet yet the soil is a very dark brown which may make it seem that way, its more of a slightly damp consistency throughout and has been like this for about 5 days without watering, which makes me question if it's simply not getting enough heat to get rid of the water.

Once again thank you for replying