Well, spoke to SIL last night.
The problem with Figaro has been building up for a few weeks now and she's still pooing outside - so that must exclude the weather as a problem.
She's also weeing bucket loads and drinking excessively so the vet thinks her kidneys could be an issue (she had a thorough examination and no bladder problems are indicated). However, SIL doesn't want to go ahead with blood tests at this stage (I may be wrong, but I get the feeling if the behavioural problem isn't sorted it's goodnight Figs anyway)
. Apparently there is already a stain on the leather sofa (I told her to get a throw).
So the situation at present is Figs is being confined to the kitchen and being encouraged to use the litter tray - except she doesn't seem to have a clue. I have passed on the suggestion regarding placement of poo inside & she is going to mop up the wee with some kitchen roll and squeeze it on the litter too.
If that doesn't work, then I guess we try a different type of litter or soil (thanks for that tip Liz)
I have mentioned about the Feliway and sent a link to Vet UK, but was told this was rather expensive.
(I shall probably end up buying one and sending it myself... or there is one in the auction.)