Do you have a link to this requirement please John?
I have seen people advise the use of fast recovery cylinders, but never
stipulated as a requirement for Building Regs!

L1 compliant cylinders must have a certain amount of insulation and a
certain size of heat exchanger (so the boiler can heat it quickly and turn
off, reducing primary circuit losses). A true rapid recovery cylinder will
actually have a much larger heat exchanger than that mandated for L1,
although the L1 is much better than the standard cylinders installed 10-20
years ago.

I believe you only need to fit one when installing a new one, or replacing
an old one.

Christian.