The problem with advertising it to OMOC members only is that your audience is very small & most of us have already got a Manta or even a collection of Mantas !
I would advertise it on a website such as this one, its free & you can post up to six pictures.
Alternatively put it up for auction at one of the well known auction houses e.g.
You will reach a larger audience if you advertise it on the Manta fan club Facebook page
Insurance valuations on Mantas are very easy, achievable sale prices are more difficult to predict.
As a general guide in the condition that your car is in
Manta B2 Exclusive Coupe £15K +
Manta B2 GTE Coupe £12K+
Manta A SR £12k +
Manta B1 SR coupe £10K +
Manta Hatches generally achieve 35% less than their equivalent coupes, this is simply down to supply & demand.
Please bear in mind all of the above is only my opinion, just like an Estate Agent i don't set the market, merely study the market & observe the trends.
Finally Stu if i were you & did not "have" to sell my car then i would keep it