Remember that contracts are a two-way thing: they bind both parties who sign them.
If you have to give 3 month's notice, then the very least I'd expect is that your employer would have to give you 3 month's notice as well. Anything less is unreasonable. By and large English law protects people who have been duped in to signing away basic or reasonable rights. I'd guess that if this had to be tested in court because of a dispute between you and the employer, equality would enforced. Of course, as someone else pointed out, if you've broken the contract by being drunk, or fighting, or whatever, then they have the moral high ground over you and out you go that day! I have to give three month's notice to my company, but in practice this doesn't mean I have to serve that period. I guess it depends on what relationship you have with your manager/company/HR but some can be very flexible. Sure, perhaps they could insist on you serving every minute of the notice period, but what if you misbehave during this? What will they do? Sack you? Without knowing any details I'd look at this as an improvement in your employment contract.
Hours worked and 'average' hours worked are tricky. So an average of 48 hours per week has to be achieved, but how is it measured? Monthly? Quarterly? Over your entire career? What is the maximum you can work? If you can't work more than 50 hours (say) then your flexibility for working less than 48 is pretty limited. On the other hand if you can do a double-shift one day, does this mean you can take a whole day off every week?
You mention this is a takeover situation. There must be other existing employees who have this same form of contract. Try to contact some of them and establish how it works in practice. Once again, the law will always take in to account what has been created as the common standard even if that's not what is written on paper.
Finally, most reputable companies really do not want to take out lawsuits against employees or ex-employees. It's expensive, unproductive, bad for moral inside, and bad for publicity outside. I can't imagine they are creating a situation for themselves which leads to this.
regards
Ken