I have a mount that wont fit my bike properly as the downtube is very, very high.

I saw a pic online of a mount like mine holding onto one of the crankarms on a similarly akward bike.

Would you do this or not trust it?

.

Well you could do it and it may hold, but you'd be shagged if anything happened as I doubt your insurance would pay out.

How about swapping it for a fork mounted one?

Looks like I may have to. It's a sod as the problem is the clamp is designed for a round downtube and mines square and fat. At the moment its inner tube and bungees. There is one that will fit but it's another 90 quid.