Hi
I believe the important date is the date you entered into the contract and not the date you pay the balance.
The business selling you the conservatory should be aware of the rules regarding VAT and they should have made you aware if there was any issue.
If you want to be extra cautions, you could phone the tax office, see this link. http://search2.hmrc.gov.uk/kbroker/hmrc/contactus/search.ladv?sr=0&as=1&cs=ISO-8859-1&sc=hmrc&sf=&sm=0&nh=50&ha=34&tx0=49685&fl0=__dsid:&tx1=2037&raction=view
I believe that only 15% VAT rate is applicable, but if this is wrong please post back.
Huckster