I'm thinking of getting some different seats, hopefully leather. I imagine the front ones are fairly straightforward but what about the rear, are they easy to replace or is this a bad idea?

Straightforward also.

Most difficult are the handles on the doors, if you want to replace these also. Be carefull when removing the panels so the corners do not brake.

Rear squab is held by 2 allen bolts underneath.

For the back rests you'll need to pull down the interior boot trim and undo the 2 small nuts holding the driver's side. It's also possible to remove the two sections without removing the rear quarter panel cards/trims by undoing the centrally mounted bracket - with this done the seat retaining pins just pull out of their sockets.

Respace, hi, what leather seats did you find that fit into a coupe? I’ve heard of people using Hyundai coupe seats but they need a base plate? Thinking about replacing mine but couldn’t find anything except 2nd hand ones. Ta

These are 2nd hand ones from Ebay, typically with Ebay they are not straightforward as part of the mechanism is bent. There are several threads about fitting Hyundai front seats and making a bracket, but not sure about the rear.
Alas after yesterday's experience my attention will be elsewhere, maybe a new engine or source parts for mine.