Thats what I would have thought. It looks like there are plenty of volunteering opportunities available though (at least in Northern Ireland anyway):- http://www.nidirect.gov.uk/volunteering-in-hospitals
They don't check everyone, only some people and your asked to declare if you have a record - whats to stop someone from just not declaring it then?
They do check everyone. It would also be an enhanced CRB check, which would disclose anything even remotely questionable or relevant that was known to the police, including being questioned, arrested or suspected without charge.