Ceri JC

Here are your options, all from memory from when I looked into it a while back, so prices may not be up to date.

Front:
Springs and oil £70
Racetech Internals £200
Matris fork internals £280
GSX-R front end swap - Cost hugely variable based on whether you keep original parts or sell them, how cheaply you pick up the gixxer bits/the condition they are in, etc.*

Rear:
GSX-R/ZX6-R/Similar shock - £10-100 based on how urgently you want it, condition, etc.*
New "budget" Hagon shock - £230ish
2nd hand Ohlins/similar off ebay (often false economy once a service is factored in)- around £250
Ohlins/Nitron/whatever - £350-450

I'd choose from the above based on your budget, giving priority to the bit that causes you most grief (sounds like rear shock).

*Whilst all might have insurance implications, these are more likely to result in insurers hiking your premium/refusing to insure you, so you might want to get quotes to see what the "total cost" of these mods are when working out how much it costs.