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Depends dunnit. You could offer them a flat fee to buy the rights to the stems off them. How much depends on how much work they put in, what level they're at etc. If it's an evening of recording maybe a couple of couple of hundred quid? You'd have to have the sign a contract to give you ownership of the results to make you free to do whatever.
Or you could offer them a writer's credit on the finished piece which you would have to register with PRS. they'd have to be registered too to recoup any royalties that would accrue and be collected by PRS. Depends on what you agree really.
Or you could do both. They might accept a lesser fee if they feel there's a good chance they might recoup later. I'd suggest you should always pay the musicians up front for their work though.
Fest' are you referring to what royalty to give her if anything she provides gets made into a commercially available track?