My mother in law had her windows replaced recently by a company which i thought was reputable, they certainly in my opinion charged well over the odds for the windows, so you would expect high quality given the price. In fact, the price for the two front windows and front doors, was more than double what another firm charged my parents for their whole house!!.
Fortunately, the windows were paid for on credit, so section 75 Consumer Credit Act 1974 comes into play, so watch out barclays, cos if i dont sue Zenith then i may sue you on the mother in laws behalf!!!
When i visited the mother in laws, i noted that the windows didnt look right, as you approach the house they looked lop sided, but that wasnt the problem, i wish it was, the windows fitted soo badly, that there was internal trim on top of trim, it looked appalling. Whats more, a lack of care had been taken installing the glass, as the gasket had been pinched and the windows leaked water through the pinched gasket.
But the problems dont end there. the window sills were supposed to be Upvc, they were fiber board, and they were sooo badly fitted,k the mitre join was so far out that you could fit your finger between the joint.
It gets worst though.They also cut the sill too short, so instead of doing it properly, they filled it up with sealant, now a small gap yes, but sealant does not bridge a 3″ gap for god sake, trades men? morons more like, because only a moron would do such poor standard of workmanship.
But it dont end there, oh now, not content will bollocksing up the windows, the window sills, then they decided to cock up the dado rail which my mother in law has, they cut it off, six inches short for goodness sake. It now looks appalling, especially cos they cut the other side only 3 inches short.
Then theres the sealant all over the walls, the broken bricks in the bay window that need replacing, the list goes on.
I must also point out that the keys to t he windows were not handed over, instead, as good client care, they left them on the neighbours wall!!!!!!!
Still, as my mother in law pointed out, shes not overly worried, because they will put it right, and if they dont, well she knows a darn good lawyer who will take them to task.
But my advice, if youre thinking about Zenith, think again.
Consumer Credit Litigation, Banking & Debt Collection 
Have he same problem I would not pay then when they replaced the windows it was still a poor job now they want their money and are threatening court.
How did you pay for the windows? did you pay for any part using a credit card or finance from the window company? If so you may be able to seek redress from the creditor.
This is a summery overview.
This all started Jan 2014, part paid for by credit card, another 25% by cheque after fitting (t was dark then but we had already noticed problems and would not sign finance agreement to say was satisfactory, we did try to start ball rolling about the credit side for redress but in the meantime had a solicitor tell us we had to let them try and rectify it. Site visit, they agreed remake due to sizes being wrong, also they put on the report that balance after inspection rice to be agreed.when they then messed it up again, over several visits to sort it and t was still not right, we had a site visit and told them we wanted or money back as well as giving hem list of problems, that was feb 2015, we then heard nothing until oct 15 and had family problems at that time, very stressed and told them we had enough site visits and re sent them the problem list. then we ended up with letters chasing the other 50% and then threatening court action.
If you paid using the credit card, then you could contact the card provider and ask them to intervene under s75(1) Consumer Credit Act.
Banks are well aware of their obligations and often have bespoke teams to deal with such things.
Its worth giving the credit card provider a call