John,
As long as the visible condition is good, the barrel seems straight/isn't rusted when you look down it, the grips aren't broken and it cocks and de-cocks without problems.....
£70 seems like a good price.
There's no anti bear-trap on them (well there isn't on my straight grip) so you can do some tests even if you don't fire it.
My mark 1 straight grip looked terrible when I bought it. Absolutely horrible in fact and yet it cleaned up really well.
Bonus to not have hours of cleaning and scrubbing to de-rust it!
The ones for sale on gunstar are a lot more than £70.
Best
Gene