Thanks for the reply 3FG.
I think I'm falling at the first hurdle in that the 7557 doesn't appear to support the setup code Video 0059. When I try to assign this code to a device button I get single slightly longer blink of the device button. (When I assign a 'proper' device code then it blinks twice at this stage).
I tried programming the individual keys anyway and the 7557 seemed happy enough to accept the codes - but no luck in actually using them. I guess this is down to the Video 0059 code not being accepted.

Any further thoughts would be most welcome.