Next year I am looking to change my 1.2 TSI fabia for either an octavia or leon mk3.

 

When looking there are far more diesels available to buy used than the petrols. The problem is I only do around 8k miles per year, this is mainly b roads and some in town. Is it completely out of the question that I go for a diesel due to this? I used to have a mk2 leon which often had to regen but perhaps the newer ones are less prone to this?

There are more diesels, because the petrols get sold.

 

I would avoid a euro6 diesel (with all its sensitive emissions equipment) for 8000 miles, the exception would be if you do journeys of at least 35 minutes, then might be worth haggling a bargain price for a diesel (remember they are distressed stock).

 

If you do any short journeys and get partial regens, all sorts of parts may need expensive replacement.  Therefore I would seek out a petrol on internet (and move fast when one comes available, as good ones at sensible prices don't stay unsold for long.

With such low year mileage only petrol is worth considering.