11 hour rest break, does this have to be in one “chunk”?

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DH is on call once a month, where he has to go to emergency call outs (heating/plumbing) at any hour of the night for a week at a time. And also has to work his normal working week (mon-fri, 8-5)

There’s currently a disagreement about his 11 hours rest.

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He finishes work at 5pm, get called out at 9pm for two hours, then again at 1am for 2hours.

He wanted 11 hours solid rest from 4am ( end of last callout) so went into work to start at 2pm.

His boss say, no, the 11 hours rest started from 5pm, and was in chunks. Eg 5pm-9pm (4hrs), 11pm-1am (2hrs) and the remains 5hrs was from 3am-8am. So he should have been in work by 8am....

Now, this means that potentially, he isn’t getting a proper sleep period for a week. He has to drive to locations and is making potentially safety critical decisions. (Gas safety related)

Who is correct about the rest (if either are)

www.gov.uk/rest-breaks-work says:

Workers have the right to 11 hours rest between working days, eg if they finish work at 8pm, they shouldn’t start work again until 7am the next day

I suspect that your DH's work would be exempt from the 11-hour uninterrupted break rule and instead he would be entitled to 'compensatory rest breaks'

"Compensatory rest
Workers may be entitled to ‘compensatory rest’ if they don’t have the right to specific rest breaks. Compensatory rest breaks are the same length of time as the break (or part of it) that they’ve missed.

A worker may be entitled to compensatory rest if:

they’re a shift worker and can’t take daily or weekly rest breaks between ending one shift and starting another
their workplace is a long way from their home (eg an oil rig)
they work in different places which are a reasonable distance from each other
they’re doing security and surveillance-based work
they’re working in an industry which is very busy at certain times of the year – like agriculture, retail, postal services or tourism
they need to work because there’s an exceptional event, an accident or a risk that an accident is about to happen
the job needs round-the-clock staffing so there aren’t interruptions to any services or production (eg hospital work)
they work in the rail industry on board trains or their job is linked to making sure trains run on time
their working day is split up (eg they’re a cleaner and work for part of the morning and the evening)
there is an agreement between management, trade unions or the workforce (a ‘collective’ or ‘workforce’ agreement) that has changed or removed rights to these rest breaks for a group of workers
The total rest entitlement for a week is 90 hours a week on average - this doesn’t include breaks at work, which are additional."