Update....
I realised that the only options are either bonnet respray, or a replacement second-hand bonnet.
The former involve potential paint issues if the job is not done properly, the latter involves potential gaps and awkward shunt-lines if the replacement bonnet is not adjusted properly.
I decided on a bonnet respray.
However...
I thought I'd use this as a training opportunity, I know the theory behind painting body panels, and have seen this done many times, but never did it myself.
So I armed myself with Halfords primer, paint to match MB 197 (Obsidian Black), and clear lacquer, and did it all by the book.
The result is pretty miserable, the paint reacted with the primer and all that has been achieved is a flaking paint and very badly pitted primer showing under it. I sprayed a larger area of course, and there are no issues with the paint on the 'healthy' parts of the bonnet, but where there was exposed primer before respraying the results look like sulphuric acid has been poured on it. I tried it twice with the same result.
The only thing I would say for Halfords, is that the primer was sitting in the boot of my car for a couple of years or more, so may have been damaged through heat or cold etc and no longer fit for purpose. The spray paint and clear lacquer were new.
But other than that it is poor show for Halford's products... being a novice I simply followed instructions to the dot and did everything by the book (or, rather, by the can - following the instructions on the label), no short-cuts etc.
I don't mind too much really as the bonnet is due for a respray anyway, and, and it was good fun, but I am still disappointed with the lack of success I had with Halford's paint and primer.