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I have a heavily circulated Lady Lavery 100 pound dated 1975, and when I hold it up to the light, I can't seem to see a security thread, only a heavy fold. However, the watermark is still there. There doesn't appear to be a difference between the space where the security thread might be, and other heavily folded areas. Any solutions/ideas?

No, it certainly isn't counterfeit - the metal security thread was only added to the £1, £5, and £10 denominations so you won't see them on the £20, £50 or £100!

@Series B notes

You need to have a look through the web site and read about the notes, visit the A Series Types page to familiarise yourself with the characteristics of each Type.

There were no security threads on the higher denomination notes. Also, there are no known instances of £50 or £100 contemporary forgeries. The only forgeries of these notes have been recent using colour copiers/printers, so called 'replicas'.

The note in the picture is genuine.