Hi Rich P, welcome to the forum. Do not ever be afraid of being a Newbie and not knowing something, that is the sole purpose of forums like this, tired old guys like me are too old to do the work so we love telling you younger fellas how to do it!
I am no expert on the Lister D but I am old enough to have worked on a lot of vintage engines with magnetos, a D is only a single so we don`t have to consider the distributor cap, therefore there are only seven basic problems that can eliminate your spark:
1. burned out magneto coil. The worst thing to happen and the most expensive to fix. Fortunately you report getting a Buzz from the High tension lead, so we can probably rule that out.
2. Failed High Tension lead from the magneto to the spark plug. This is very common as the old rubber insulation deteriorates with time. Try cranking your engine in the dark and watch for sparks running up the outside of the HT lead. If this is the case replace the HT cable. (cheap and easy)
3. Worn points or incorrect points gap, this is unlikely as your engine was running well previously. It is common for the points to corrode in a wet/damp climate. Clean them with a piece of fine emery cloth. I do not know what the points gap should be for your magneto but I would guess at 10 to 12 thousands of an inch. If you do not have feeler gauges then A4 paper is generally between 2 and 3 thousandths thick, so three strips is too little and four is too much, four strips of paper with a little pressure should get you in the ball park.
4. Condenser failure, these are an electrolytic capacitor and have a finite life. When they begin to fail you end up with a diminished spark and eventually no spark. My best guess is that this fits well with the problem you have. A replacement should be available from Stationary engine parts in the UK or any number of online web sites.
5. Timing, I very much doubt that the spark/ignition timing would have altered since you last ran your engine unless you have been tinkering with it. If you have please let me know and I will try to walk you through resetting it.
6. Corrosion, it is very common to get corrosion between dissimilar metals IE aluminium and steel, for a spark to work it must be able to return to its source via the chassis of the engine and back to the magneto. If you have rust/aluminium oxide between the two then your spark will deteriorate.
7.Magnetos are so called because they contain a magnet, it is possible but unlikely that the iron core has lost some of it`s magnetism. It is possible to re-magnetize them but this requires specialist equipment.
Hope this is helps,
Bob