It's not the 'common' sow-thistle as these are yellow flowered - Sonchus asper and Sonchus oleraceus.
According to the RD book of wild flowers it could be the Alpine blue sow-thistle (Cicerbita alpina) on the following on Wikipedia.
https://en.wikipedia.org/wiki/Cicerbita_alpina

Again according to RD "Very few colonies of this tall. stout-stemmed plant are known to exist in Britain. It grows on moist rocks in north-east Scotland. 22-80 ins. (55-200 cm) Flowers July-Sept."

I would be very surprised if it was cicerbita alpina which is a very rare native of Scotland (not Surrey). according to Plantlife.org.uk :
"Alpine blue-sow-thistle is a very rare plant in the UK: it grows on only four rocky ledges sites in the Grampian Mountains of eastern Scotland."

Cicerbita plumieri is the genus of the blue sow thistle. I added the species name to the other thread too.

If you like it and don't want it to spread as much then dead head it after flowering so it doesn't set seed.

Thank you very much for your help. I am going to reduce the number of plants and will deadhead as suggested.

Thanks Seaburn and Urbanite, I couldn't think it would be the rare one, but didn't know of any other. I have now looked at what you said Seaburn and it's really pretty - obviously not for the garden if it grows that quickly, but on my field would look lovely! I shall look out for it.