Just had my rear tyres changed and took the chance to check over the rear brakes etc. Noticed that the discs look like they may be developing cracks and also the drivers side inner guard has broken at the rear of the disc. Any advice on replacement discs and where to get a replacement guard?

If I read it right you are saying there a also a cracked brake disc. If so the car is dangerous and should not be driven. A full set of discs and pads costs peanuts so replace the set before the old ones fall apart and lock a wheel.

Deep rust pits are also a risk for cracking. Again the only fix is new discs and pads.

The inside cover/guard is a piece of tin to supposedly stop road debris getting into the brakes. But in reality it can trap stones between itself and the disc making an alarming screech.

IMO the tin cover can be removed and ignored but a new part wont be costly.
Bikes dont have the covers and bikes dont get grit jamming their brakes.

You can buy expensive drilled and slotted discs that look cool but there is little point in upgrading the back brakes. Making them too good, simply activates the ABS, so you'll never use their potential stopping power.

Tarox full fat slotted drilled etc etc - £140
EBC do a pair of pair of plain for - £70
Delphi have a pair of standard rears for £20 (the same as used on lesser models at the front).

Rear pads are about £15 a set.

If you upgrade the whole car, then better rear brakes might be worth having, but I believe even then there's no functional benefit in more power at the back.

Talk to Shop4Parts for a set of discs and pads.

You will need a wind-back tool for the hand brake cam and take a close look at the sliding pins and caliper seals.

Caliper rebuild kits with seals and piston are around £20 per side. Slider pins are another tenner.

Someone on eBay is selling pair of 500 Abarth / Panda 100HP rear callipers for £200. Red powder coated, etc, etc.

Thanks - it is pitting with black lines radiating so not a crack I could feel but rather concern that it will. Looked around at parts and don't see the point in slotted/ drilled disks as the car has standard power. I am going with better than standard pads just based on past experience.

If you use uprated pads then put the same on front at back. Its not a good idea to update the back only as that could cause a wheel to lock (or trigger ABS) when normally they would not. Uprated front only is less of an issue but again best to keep all the same spec.

The fancy rear discs look cool but at many times the price of plain discs they start to look silly money.