In reply to leon:
Does the sump you dug have a drain? If not, isn't it simply going to fill with water up to the level of the water table, and then you have the same problem? Or are you assuming the sump will drain into the surrounding earth (sounds like that's the plan your Dad is suggesting by digging under the floor)? What's ground type? Are you hitting impermeable bedrock or clay (i.e. so the sump cannot drain)?
I take it you meant 'visqueen', since google doesn't find much relevant for 'vasquine'...
http://www.visqueenbuilding.co.uk/
Maybe one of their tanking membranes might be the answer, but that might involve digging out the entire space behind the wall, applying the membrane, and then back-filling.
http://www.visqueenbuilding.co.uk/tanking
ps. I'm just working from the basic principles of the problem, not from any professional or personal experience, since replies seemed rather sparse...