After around 30 mins, got out when I noticed the front was sat very low. No warning light came up and nothing since has come on.
I dropped the car to low and it settled on low. Came back up to road and now won't go beyond that, or below that. When trying to flick the switch to lower, there is a couple of flashes like when trying with a door open. When trying to raise the car, if flashes near constantly, but does nothing.

Short of taking it to a specialist in north Kent, is there anything to do or check before that without a fault code reader ?

Detached suspension sensor arm or air leak for visual and aural checks. 2007 Vogue TDV8 in Zermatt Silver with Charcoal

My leak was only audible when I set the suspension to off road (high), it was then clear to hear.

Do you have GAP tool? it was very useful for monitoring compressor performance, air pressure etc. BMW i3 Electric Car
2012 Full Fat RR 4.4 TDV8 (now gone)
2006 VW Touareg 3.0 TDi V6

Most likely what's happened is the bag has split as it folds in on itself at normal and low height and wears over time. When you go into extended mode this part of the bag is stretched and exposed and eventually might fail with a loud bang like mine or just leak. Bags are easy to replace on the front with the right tools.

If you can get it into extended mode you will probably hear it hiss or leak air if it won't inflate at all try jacking that side of the car up and see if you can see a split in the bag as the weight of the wheels etc will extend the suspension. 2011 4.4 TDV8 Autobiography