I started keeping an eye out for bits, and found an old broken toy that I liked the look of, and worked out was one of these (on the right)
The size was good, the face looked suitably robotic and '80s-looking and it had overtones of M.A.S.K., which was no bad thing.
I nicked the legs from one of these (in the middle) that I'd found on eBay very cheaply from Singapore.
I stuck them together with a pelvis made from two 25mm hex bases, and added two Assault Cannon barrels left over from when I converted my Space Crusade marines to Lascannons.
After the usual quick paint job and varnish, I'm pretty pleased!