There was a case, IIRC involving an ICI pesticice, that held that instructions were part of the product and faulty instructions meant that the product itself could be regarded as faulty.
If you bought the tray as a consumer I think you could expect it to come with sufficient instructions that it could be fitted by a competent DIYer or professional. Therefore you may have recourse against the retailer if you can show your plumber followed the inadequate instructions and it all went wrong.
Owain