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. It doesn't matter if
you are no good at plastering - you'll be tiling over it. Get some plaster,
how much depends on the size of the gaps, and having distressed the wall to
provide a key, put it on level with the back of the worktop, tapering the
plaster up the wall. One coat should do it, and when it's gone off a bit
(say 1 hour) level it at much as possible.

Rob Graham