I would say £850 is your ballpark.
The return spring, hop bucking & nub along with the furniture will only add so much value as they're replacing rather than adding to the stock items. Especially with the stock and grip, if you had the originals, you could boost the value by at least £50 as they'd be listed as an addition instead of a swap.
I'd say the sling and handstop (as you said it was cheap) would be where people may try to haggle on the price so I didn't factor them into my valuation.