Poor spark on Lucas SR1

I have recently been having a bit of a struggle to get my Lister D to fire up. It doesn't get a lot of use but I try to get it going a few times a year to keep it alive.
I seem to be getting a pretty weak spark when turning it over with plug held to the engine metalwork. Probably even worse when it's struggling under compression. Can anyone suggest what I can do to improve matters?...........starting with the cheapest. A rewind is well beyond my means so not an option.
Thanks in advance.

You can often find SR1 mags quite cheaply on the tat stalls at rallies if the Bakelite end cover is missing.
Take a multimeter with you to check the coil, you should get about 4000 ohms.

Sorry, as you live in France this reply may not be much help  

No help at all but thankyou for trying. The French would want more than the cost of a coil rewind for another S/H magneto. And the handful of rallies, etc. are too far away, etc. I was hoping for some input about what might be causing the weak spark. I will check the resistance of the coil as you suggest. I assume that is between the brass button under the bakelite cover to earth? Or is it to one or the other side of the points? I don't really have any sort of automotive engineering background so pretty much clueless when it comes to the basic operating theory. If the resistance checks out OK then maybe a condenser. An old one from a car might be OK if it physically fits the space.

Update. I was reading an article about renovating a Lister D and the author said to avoid painting the platform that the magneto sits on. There needs to be a good earthing connection from the body of the magneto to the engine metalwork. I decided to carefully lift the magneto and take a look underneath. The bolts were filthy with rust and muck and the surface under the magneto had about three coats of paint on it! I took a sanding disc to that lot and cleaned up the bolts as well as I could. The magneto provides a much better spark now and the engine runs sweet.

Tres Bien! It never takes long to find a solution or advice online!

enjoy your engine running better!

Hamish