in the past with similar salvias I have just trimmed it back with shears/secateurs, to the height I want. then any none flowering stems are put into compost to form roots and new plants. these are kept frost free and planted out next spring.

Agree, don't be shy about cutting them. They can handle a harsh pruning and will often rebloom. If I just get tired of looking at it, out it comes roots and all. No regrets.

Thank you both. Should I do it as soon as flowering is over, say, September? I like the idea of growing some new ones to sell at our Horticultural Society plant sale in May for their funds, so should I tae the cuttings now?
By the way SBG, the yellow foxglove you gave me last year bloomed well and came up again this year with a larger spike but cream rather than yellow.It's been lovely, thanks.

that's good re the foxglove.
i'd do the salvia now so any new growth can harden off before the frosts.