Thanks for the tip. I did test for the spark when cold and got a good healthy one; I didn't think of testing when hot, but now that might be a bit difficult as the engine won't run for more than about 30s - a minute. As you and others suggest, the coil could well be a problem as well or instead of the carb.
I've just got another quote from a UK dealer to supply (not fit) a new carb at a cost of £156, that's $235!
And yet in the US this one for example is one fifth the price.
I don't understand the price difference; if anyone can shed some light on it I'd be interested to know the answer. And the US seller can't guarantee his carb will work on a UK model, so it looks like I'm stuck there.
As to whether Stihl are going down the pan, all i'll say is that in the last year I've had the KM55 go on me, a KM56 (leaking fuel tank, faulty gas cap and coupling that overheated), an HS45 hedge trimmer (crankshaft bearings), and a blower attachment (KM-BG) on which the drive shaft shattered into pieces breaking the aluminium sleeve into which it inserts right in two. Why Stihl couldn't make the sleeve out of steel I don't know - maybe they wanted it to break. A dealer told me I shouldn't have been using it commercially as "it's not a commercial grade item", but my answer is it was used only very occasionally, and it's news to me that whilst some of the engines in the Kombi range are billed as commercial grade (e.g. KM130R, which I've got) some of the attachments are not. I don't see anything in the Stihl literature pointing this out.
What I do know is that after using Stihl stuff for nearly thirty years, their durability is not what it was.. I'm seriously toying with going over to Shindaiwa or Echo, but I've not yet researched availability of spare parts here in the UK, or more importantly spare part prices.