Rogue have informed me they have passed on my enquires/problem to Lee (the material manufacturers). So they're interested.
I'll feed back if/when I hear more, though I've been informed it will be after the 6th Jan.
This site in the UK
Lighting Gels / Lighting Filter Colours | Stage Depot sells a 61x50 cm, high temp sheet of gel for around £10 plus p&p. This will be way forward. My current plan is to use a diffuser cap, or similar, and replace the diffuser part with the red gel, maybe with ventilation holes, making a simple to use cap when required. Could use different strengths. Or I might just keep it simple and tape over a large bubble's worth of gel, with ventilation holes - at the price, it's going to be cheap to experiment.
I've managed to remove 99% of the red tint from my flash head with a citrus glue remover. I can just detect a trace remaining when the light catches the complex lens pattern etched in the flash head.
Almost got to the end of this little saga. I'll report back when I hear from Lee.
Happy New Year to you all ...