Is it a legal requirement for a builder to supply a certificate for tanking.

Hi, Legally responsible? No. You need a guarantee certificate. There are typically two types of guarantee, with soveriegn, kingfisher et al. One is insurance backed, expensive usually 3-4% of total project cost but covers you completely, the other is a dual guarantee whereby the contractor guarantees his work, the damp proofing chemicals by the manufacturer, this is usually a fixed cost of £35-£50. In either case the installer needs to be approved by the manufacturer, usually by undergoing training. A sticker on a van implies nothing, he/she may have even used cheaper products not supplied by Sovereign, so could not guarantee his work. Talking to Sovereign will establish his credentials, if he has none, use it to talk to trading standards but only if he misrepresented himself. If you made assumptions however, maybe based on a sticker, you will be at fault, learn from it and move on.