Compulsory CRB checks for anyone who has any official contact with children ever seems like a waste of time to me. Aside from discouraging adults from having any interaction with children, it doesn't seem like a particularly effective method of protecting children in any case.
There are around 16 million children in Britain, let's suppose that there are around 15 million parents of children ( a very rough estimate of course but that doesn't matter for now). Assuming that maybe 1 in 15 of them wishes to get involved with helping children other than their own that would require 1 million CRB checks each year. This is a guesstimate but it seems like a reasonable ball park figure. Each test costs around £30, so let's say the total cost of all this is £30 million (not including the cost of lost volunteers).
Meanwhile there are around 30000 people on the sex offenders register. Why not take the £30 million and add £1000 per known offender in additional funding, for monitoring the people on the register? With that money they could easily carry out a number of random spot checks on each known offender throughout the year, in addition to the sex offender monitoring that already occurs.
Since it is effectively impossible to check every single person who has access to children in any case the whole thing amounts to trying to catch water with a sieve in any case and risks creating false complacency about child safety by pretending that comprehensive monitoring is possible when it is not and probably never will be.