Set about stripping it down; fairly straight forward as all the screws are still in good condition. First thing I noticed, how easy it came apart: the gaskets seemed not to have stuck very well. The second thing I noticed, after I had removed the fan, was a nice leak coming from the bottom on the inside; which made me think perhaps the gasket wasn't making a good seal.
Next job.
Get the glow plug wire gauze out. To achieve this I found the easy way was to collapse one side and remove with a pair of long nose pliers. As you can see from the picture, it's in a bit of a sorry state.
Here are the gaskets. The one on the left still had traces of grease round the edges, the felt O-ring was absolutely soaking in diesel and the burner gasket just fell out.
By the looks of it, it wasn't doing a very good job as a seal: you can see where the gases had bypassed.
All cleaned up and ready for the gaskets. Could not believe the price of a service kit but as the rest of the heater is in good condition I see it more as an investment than something I have to pay for.
Glow plug cleaned up nicely although I don't know if I will be using it as I am getting a new one in the service kit.