"..........I've been lucky enough to be given some soil warming cable and a
couple of old thermostats ......."

firstly I would fit a plug and check if the cable is working.
I have 3 old cables not used for a few years and no sign of life in any of
them.
I was told that moisture can get in and finish the cables if they are left
in the beds and not used for a year or two.

Also remember you put the cable at 6 to 8 inch spacing so a 3ft wide
propagating bed will need 6 lengths of cable so you have enough for about 17
ft of bed altogether. Useful size.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk