OK so I bought 4, MK, Crabtree, Volex and Pro Elec. They are all pretty much the same switch mechanism which is slightly better than the one I got at B&Q which had a simpler mechanism and wasn't even greased. The two cheaper ones are actually a bit neater because the bezel is threaded on to the base where as the MK and Crabtree have a deeper pattress and have two screws to hold the switch unit to the pattress.
All have two small screws inside holding the switch unit together except the Pro Elec which clips together.
The MK was twice the price of the others but pretty much identical internally. The Pro Elec looked the cheapest but was second most expensive.
I pulled the string off the Volex after a couple of normal pulls, I guess this was due to poor tying of the knot inside the switch but it was a bit of a fiddle to take it apart, re-thread and re-tie it. That said on first impressions the Volex is probably the one I would go for based on price, quality and compactness. It was also the cheapest by 40p.
How long any of them last in operation remains to be seen.