Ypu've given them plenty of chances to correct their mistakes.
With this conservatory you should have had two things.
1. A warranty usually 10yrs
2. A Fensa Certificate to verify the windows fitted conform to the building regulation on safety and thermal insulation.
If you weren't issued with either I am afraid you may have fallen into the cowboy trap or a trap of a company of jobbing builders who haven't a clue what they are doing.
so Go to www.consumerdirect.gov.uk email them
click on 'contact us' top of page fill in the online form telling them everything and that they now won't reply to your requests to correct the leaking conservatory.
You will probably be asked for the paperwork so get it photcopied and send them copies not originals.
Personally I would send then a letter telling them you intend to take out a small advert in the local paper to where the company operates saying
Does anyone know how I can get this firm to reply to my letters sent asking them to come and fix the leaks in the new conservatory less than 1 yr old. They sent one employee back 7 times and 7 times he failed to correct the faults. They won't like bad publicity.
I'd be interested to know the outcome so post using the answer page to your question not as another question.
Bob
November 2010