Small world - I also worked for Mullards in their New Road factory at Mitcham.
I visited the Mitcham factory in around 1973 as well as Southampton, Blackburn (where I did my workshop training) and Simonstone. I also visited several Philips factories in Holland and France.
I think that Hazel Grove is the only one still going although no longer part of the Philips empire. They sold most of the business to a group of venture capitalists and it became NXP with Philips still having an interest. Philips then gave their share to the Pension Fund. (From which I am now benefiting!) The Hazel Grove factory is now in the process of a sale to Nexperia - a Chinese group of investors.
When I was at Blackburn (1970) they were still making valves. Around 6,000 worked there and their training school took apprentices from many other Mullard plants as well as from third party companies in the Blackburn area. Sad that it's all gone.
Andrew Mawson also worked for Mullard (I think at Southampton?).
As you say - it's a small world.