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in my personal experience any CRB check only applies to work in a particular place/job, ie: doesn't cover you to work in two different places who may need their own CRB check............ whilst I may refrain from using any sentence that employs the word 'fucked' I would certainly enquire as to how the CRB check will ACTUALLY effect employment...

Absolutely right. I'm responsible for CRB checks for sessional staff as well as all permanent employees at my workplace. It's a ludicrus system, in my opinion, as as soon as it's issued it is already out of date (what stopping you going out and committing a heinous crime the day you get it through the post as clear???).
We pay for all CRB checks (an enhanced check, which is what is required due to the type of work we undertake, is £60) and would never dream of asking empolyees to foot the bill themselves.
I've worked for 3 separate organisations that I've had to be CRB Enhanced checked for and not one would expect me to foot the bill (I also had to be security cleared to work for MoD years ago and this is even more thorough(=more £££), and again, didn't have to pay for that).
I'd seriously look into it. If a paticular employer wants to CRB check you, they will have the facilities to do so and will also foot the bill.