Can I keep a treadmill outside?

[Cockadoodledooo]

If I have a cover of some sort for it?

Talking basic magnetic/self powered one rather than a plugged-in one. House is v small, no garage to put it in. I can't always get away from the house to exercise (small children, dh often absent in the evenings).

Or is it an insane idea?

Can you?
Yes
Is it a good idea?
Probably not!
(unless you build a shed around it :o)

our neighbour seems to, and keeps it under a tarpaulin. i have never seen them use it though so have no idea how it's weathered!

A park nearby has special outdoor exercise equipment. First search I did found this website fresh-airfitness.co.uk.

We have the same problem. It might be idea?