It seems internally contradictory. Eg. they say there are no normal hours, there's no guarantee of work, and there's no obligation to accept the work... and they say that because basically guaranteed overtime and compulsory overtime have significant effects when it comes to holiday pay etc... whereas if it's not guaranteed and non-compulsory that's less of an issue (although some tribunal cases indicate that might change...)
But then it says they're expected to work when required/according to the needs of the business, which is contradictory.
As people have said, it seems like an idiot has written that... or they're trying to be too clever for their own good. Re: your NMW concerns, it's one of the things HMRC tries to crack down on when they find out about it...
They do that but have it overtime which isn't guaranteed or compulsory... according to the contract... then they cost less than employees on full time. An old employer tried that on with me back in the day... basically worked ~42 hours religiously, but my contract was for 30ish hours I think. Those extra hours were not guaranteed or compulsory so they basically were trying to do me out of holiday pay... but soon got that changed. TBF my boss was fine about it, but as you're obviously aware of from highlighting this issue, in practice it results in people getting mugged off in terms of holiday pay and also being too scared to moan about it because they're worried that instead of working the full time hours they ordinarily do, they'll instead be slashed down to their basic hours and be left in the ****!