How to check block isn't warped?
 

Is it just a case of lying a striaght edge on the top and looking for any gaps?

Dropped my heads off to be skimmed and presure tested after overheating problems and im parranoid about the block being warped

I would be very surprised if the block was warped mate,would need to have gotten VERY hot indeed.
What was the cooling issues?

i should of took a picture... head gasket failed to water jacket

Do the job properly & get it skimmed, even if it has'nt been that hot, they do settle & move over time, if its a 2.5 its a must if its had head gasket issues.