Sorry Sparks, very rude of me not to respond to such a query...
I did indeed get them off the other day in between other tasks that needed more urgent attention. Bought what I thought was a straightforward replacement siphon but alas, the body of it is just a few mm longer than the old one, so it was a bit of a wedge to get it into the cistern.
Tightened it all up, popped it all back together, turned the water back on, only for it to start leaking from all possible places.... toilet is currently back in bits awaiting another hour for me to have spare to get my nuts tightened up everywhere before I risk getting all wet and spurty again...
Worst case scenarios is I have to go find another model of siphon that is slightly smaller then the new one.. it was only £12 odd all told (with additional new fixing kit) so no major worries apart from the time pressures to get it fixes before the Brizzle in-laws come around on Christmas Day - the old fella needs the downstairs loo to be in operation to save him trekking upstairs all the time.