Les I don't know your tent in particular, but the general rule of thumb is that if they are taped the seams
should be waterproof.
I would set it up in the garden and shower it down with the garden hose and see what gives, you'll soon find out which seams if any are leaking. Bring it indoors, dry it off and then proof the seams if required.
As far as actual sealants are concerned there are a few which are pretty good, Grangers Seam Sealant is probably one of the easiest to get hold of, but in the age of the internet you could also get hold of others which are probably a little better.
Mountain Equipment actually include sealant with thier tents for repair purpose, it is called Silnett and is supposed to be pretty good (Info from a friend).
Silnett is made by a company called McNett who also have a product called Seamsure, either of these were available from
PointNorth under Accessories, although I think it was Seamsure wasn't in stock last time I looked. You should be able to get a good deal from them, hope this helps,
Leon.