So roughly speaking a 9 hour day shift with a lunch break and no more than an 8 hour watch if any part of it extends past 2300L.
You're right the MOD signed up to support the WTR in principle (and has since given it a stiff ignoring) Recheck your WTR - day shifts, as I recall you are entitled to a 11hr break between shifts (that's time finished work to time start work - no allowance for travelling time) which means you could do a thirteen hour or more, shift on a repetitive basis, provided you get time off between and after the cycle -SEE working week limits.
If you work more than six hours you are entitled to a 20min break, that is a break that allows you away from work, not where you are kept on standby for a PAR whilst you have a sandwich in the crewroom - (in fact if you are called for a PAR after 19mins you are then entitled to another uninterrupted 20mins).
Night shifts should not avearge more than 8hrs, if more than 3hrs is normally worked between 2300 and 0600 - so you could be asked to work twelve hours from 1400 until 0200 (the wording on averaging night working is somewhat ambigious and must be read carefully).
Working week shall not average more than 48hrs, averaged over a 17 week period. In which you are entitled to one day of each week, or two consecutive days each fortnight.
At a previous ATC unit we were very hard pushed (no names no packdrill) but even at the lowest moments we found we actually were still within WTR limits - I suggest your only recourse would be through "best practice" (i.e. quoting the CAA regulations as exemplary) or playing the flight safety card. Trouble is it will take balls to stand up to your Line Manager and say "I can't cope with this pressure" - (make sure you're no benefitting from any stand-downs - or other such freebies or you might expect your ACRs to tumble!) and to be honest you're are only likely to succeed post-incident/accident (as someone above has said).
Best bet, but finding the right case to argue is the tricky bit - might be the Safety Management Route? This is a hot-topic, and an audit that proves safety is being jeopardised by working practices would have to be signed off, and if not acted upon can be passed onto higher authority (mind you it then might become your word against someone elses!)
I'm not suggesting you aren't being over-worked; BUT do your research, gather your facts and think carefully before you act - trust me, this has all the potential for the classic "shot-in-the-foot".