I have tried the new wheel on the current chainset and it keeps
missing some of the cogs on the cassete as the gaps on them are much
closer than on the 7speed cassete.
Is this with a new chain?
1. An old chain will skip on a new cassette if it's much worn. Stiff
chain links cause similar symptoms, though.
2. Problems selecting gears and chain staying in gear can be caused by
improper cable tension or shifter/derailleur/cassette mismatch.
A chain that's a little too wide would still work but will be noisy as it
rubs adjacent cassette sprockets.
Can i get hold of a thiner chain
If needed but I'm not sure it is. But get an "8 speed" chain if you need
a new chain anyway, eg. SRAM PC58.
and if so will i then have to get a new crank?