I didn't get my conservatory from B&Q but it was from Cestrum. It also leaks in exactly the same place as yours does/did. I had a local surveyor inspect the construction and his conclusion was that the conservatory was a poor design - being reliant on silcone for weather tightness. It was NOT a construction issue but a design one. The company did not reply to the surveyor's letters and now I find that the company has gone into receivership - so much for a 10 year guarantee. So my only solution is to reapply mastic every few years and floor the conservatory with tiles making mopping up of water easier. Did you get any joy to your enquiries?