Shame you have to consider selling it, especially at the moment, as the market seems to be hardening a bit against these cars.
Although you've given a fair bit of very useful - and honest - information to help with a possible valuation range, the one key missing bit of info is on the gearbox history and condition - i.e., has it been serviced/refurbed/etc, as that's key on an '8 with this mileage.
Depending on the answer to that - and given what you've said about it needing "extensive work and the cylinder heads needing refurbed", it could be anywhere between about £2.5k and £5k, possibly a bit more if the work mentioned and any gearbox work are inexpensive.
Others will doubtless be along to give more expert views than I could, but good luck on getting a buyer for it - and I'd be interested in the subframe, if in good condition and if you're selling separately
