Sorry for bumping. took me ages to find a cost effective solution but it may help others. Volvo wanted £50 for a replacement pair. SGS didn't recognise the Part number but offered to regas for £7.99 a strut plus vat and postage. I took a chance and ordered these off ebay
https://www.ebay.co.uk/itm/172649523269 and they work fine (after an hour of not being very clever trying to fit them - leave the struts off one end until you have refitted the dog guard). Cost me £7 all in. I went for the 80n version. shame they aren't black but hey ho.
I will report back if they prove not strong enough.