Ben,
Ok, I had hoped you would have got a couple of valuations by now,and I am sure by the end of the week you will have a couple more from fellow collectors, anyhow here is my two pen' orth.
The 78 and 79g are very nice pistols and very collectable. As with any valuation it will based on personal experience of prices paid for the same model and prices do vary depending on the economy for one thing.
Going by your description I would pay £ 150 - £175
A 78g was advertised recently on UBC for £225 and was sold, but we do not know the final price paid. It is best to advertise it at the upper end of the estimate as you can always lower the price if there is no interest.
Some folk may be willing to pay £200 - £225 for it, but personally I think that is too much.
As always you will need to host a couple of decent photos of the pistol to get a definitive answer.
Always wait until you have 2 or 3 valuations before making a decision.
Good luck with the sale, comrade.
Pete.