On Wed, 03 Aug 2016 17:21:10 +0000, Maurice wrote:

Previous owners had fastened down an inspection chamber lid on the
patio,
but the screws will not budge.
Tried numerous applications of PlusGas, and it does disappear down the
sides of the screws, which later still refuse to move at all.

Apart from keeping on with the PlusGas, are there any other ways of
persuading the screws to turn?
(Last resort: drill them out...)

I've found Plusgas and all the other so-called "releasing agents" to be
totally useless, to be honest.
This looks like a job for an impact driver. I'd be reasonably confident
about getting them out with one of those, having previously had
consistently good results with them.