OK - Based on lots of helpful advice, I have the bath in now.

As it's plastic, I took the advice here and screwed 3x3" and 2x4" battens
(depending on whether there was a gap between bath and wall or not) all the
way around bar the open side edge.

Even with the feet loosely down and the bath is not siliconed in yet, it is
*incredibly* firm - I was standing in it painting the ceiling (again for
the 4th time - bloody Dulux Bathroom "no coverage" paint).

It is still to get a 2x2" frame to support the front edge and be a fixing
for the bath panel, so it should firm up a bit more still.

Think I'll run some silicon between it and the back wall as suggested (the
only zero gap side) and perhaps put a few dobs under the front and back end
edges to assist with lateral stability (the bath is not actually *fixed* to
anything), but fortuneatly the battens went in true and level so it's
seated very nicely on the rim.

I don't regret getting plastic now - in fact given the contortions required
to drop it in the awkward space, I'm bloody glad it wasn't steel - I'd be
in hospital with a hernia and several broken fingers and booked for a spine
replacement if it was.

Ta!

Tim