I'd bet most don't get overtime. For years their contract has been (e.g.) 2000hrs/year, we'll tell you where and when. No overtime, e.g10% uplift for working weekends.
There might be a few ex-HMCE staff on the Cutters and CPVs on legacy AAA T&CS, but it would be no more than that. Almost all will be on AHW i.e. c.2,200 hrs pa with up-front payments for flexible changes, nights, etc.