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You want a 720nm for the best effect on films available now, higher and it'll be hard work getting an image and higher than 760 and you won't get one. (I haven't tested but this is what data sheets and people on the internet say.)
I use a Fuji gel filter cut to size and mounted in a regular filter with the glass taken out (it was cheaper than a regular filter and probably better quality than most of the not super expensive ones). Don't know if they're available/cheap in the UK but maybe there's something similar.
Thanks, this was Rollei Infrared but Rollei Retro 400s and Rollei Retro 80s are equally IR sensitive and they're a bit cheaper (and I suspect Infrared is the same film as 400s with a different label on it...)
I think those are the only films in production with real infrared sensitivity. I have some Retro 80s to try IR with as soon as I have a chance.