I do not have very much experience with people leaving prison so forgive me if this is a really stupidly obvious question!

I can see from the CPAG handbook that IB or ESA are not payable when someone is serving a custodial sentence but does this mean they invariably have to re-apply on release or can the existing claim simply be re-activated?

I the olden days, IB could be reinstated at the same rate.
However a new claim for ESA must be made in these modern times