hehehe update - I was outside Toolstation this morning at 9am.....unfortunately the Toolstation folk weren't. Never mind, by 9:30am I had a nut splitter for a fiver (2 actually).
Anyway - this became a tale of four nuts.
I threw the works at the nearside top nut. Eventually a combo of nut splitter and drill did the job.
The nearside bottom started off intransigent, but a sharp tug with a 17mm socket spanner did the trick. I had to cut off the rubber at the back to get mole grips on to stop it spinning. Easy.
Offside bottom went the same way until the last half dozen threads - which had basically knacked. Approx 8 yrs old, mind.
I whacked the splitter on it, and made a deep bite - turned it around and did the same on the other side. Off it came, sweet as a nut.
The offside top promised to be awkward as the spanner started to round it off. I made a cut with the splitter and I can't remember why, but then I tried the open end 17mm spanner, and it just started to twirl. Had to cut the rubbers again - but easy enough.
New links were relatively easy to install. Test drive 6 miles - lovely.
So, bit knackered, and a bit wiser. And a little bit less skint.
Thanks to all for their sage advice and generous offers of assistance. I love you all.