I see there are chances of thunderstorms late next week showing up on the models.

I don't normally associate thunderstorms with September - how common is this?

Does anyone have figures on average days with thunder per month in the UK?

Don't know about averages, but you can certainly get dramatic thundery weather in September

http://www.metoffice.gov.uk/media/pdf/3/2/Southeast_England_Floods_-_15_September_1968.pdf

Thunderstorms are fairly common in September as there is still a lot of heat to our South at this early stage of Autumn to push them our way if conditions are favourable which looks possible late this week. Probably going to be a bit hit and miss though.

September 1992 stands out in my mind as well - Was a Thurday night/Friday morning!

Certainly was - I was living in Bromley at the time.

Really more like a long sequence of Thunderstorms interspersed with heavy rain lasting from about 11PM on yje Saturday to 5PM on the Sunday

Goes down as one of the top weather events in my memory

While I think of it, here's another big September thunderstorm

http://www.ukweatherworld.co.uk/forum/index.php?/topic/47563-5th-september-1958-hailstorms/

The same storm holds the record two-hour rainfall for the UK of 131mm, at Knockholt, Kent. 

I can remember this one, living at Orpington,  only a few miles from Knockholt, at the time. Low yellowish clouds moving very rapidly, continual lightning and torrential rain which caused flash floods and tookvast amounts of earth of fields and into roads. For those who know the area, Addington Lane was about 3 inches dep in silt the next morning. 

But the most amazing thing was that after a couple of hours of downpour, the wind having been strong SE, it suddenly dropped and a small patch of blue sky appeared. Half an hour later, the wind was back, but from the NW, and it was raining hard again. I've been told that this sort of thing doesn't happen in Britain - but I saw it!

Some storms rumbling away in the Channel this morning Northwest of the Channel Islands with developments extending into Cornwall and Devon soon.

You did. A few storms quickly developing SE of the IOW - recent sferics. A few more into Cornwall.

The action ramps up later with a line of storms developing across the South from midnight.

The satellite imagery this morning sure seems to suggest that thunderstorms may develop somewhat more readily than forecast today.

Euro-4 continues to show little activity until tomorrow morning, yet already a new cell has formed near Portsmouth and there are a few areas of new convection building across Northern France too.

Ok thunder to my South here - shotgun 'canon' waveform indicating elevated base (the percussion reaches you directly from above and is cleaner without interference from topography, etc). Every 20s or so at the moment.

Torrential rain started here - very large drops - from a side-shooter developing to the NE of the main cell.

http://www.meteocenter.eu/fajli/index.php?domain=euNMM12km&region=EU&forecast=KPrecip&time=21

Like Euro4, WRF has totally missesd the Solent storm, but unlike Euro4 it develops some cells across Southern England this afternoon.

http://www.meteocenter.eu/fajli/index.php?domain=euNMM12km&region=EU&forecast=KPrecip&time=24

This looks interesting but implies that development doesn't occur soon enough to affect locations as far south as where I am - but this morning's storms give reason to hope that storms might develop more rapidly than shown.

Tomorrow looks even more interesting:

http://www.meteocenter.eu/fajli/index.php?domain=euNMM12km&region=EU&forecast=KPrecip&time=45

An organised band of convective precipitation advancing north. Fizzles out during the evening though.

A part of me was hoping for a totally dry month to go in the record books, but it would take something extraordinary to shut out my enthusiam for convective storms!

The links in my above post don't seem to be working 

http://www.meteocenter.eu/WRF-forecast/EU/Europe.html#model

Have a look under convective precipitation here and you'll see what I was on about 

Interestingly different from the MetO, which has earlier development and then clears the storms away northwards into the Welsh borders rather than having them sticking around the M4

http://www.metoffice.gov.uk/public/weather/forecast/map/gcp3nqsgd#?map=Rainfall&fcTime=1411095600&zoom=8&lon=-1.10&lat=51.65

Just had a flash of lightning and a clap of loudish thunder, strange we had nothing on the radar half an hour ago so looks to be building over us, no rain as yet though.