robgraham wrote in message ...
But now that I have large gaps how do I seal the worktop to the wall or
should I forget this and just tile, bringing the tiles out from the wall
with large lumps of grout to cover the gaps.

The best way is to plaster the wall to fit the worktop.
Quite often there are just a couple of high spots on the wall that throw the
worktop out. Chopping those out can reduce the gaps to tile thickness.