sheet metal design advice please

Dayton Rogers has a decent (free) design handbook available on their Web site and I think you can read it chapter by chapter. You have to fill out a little personal info, however. If you were in the States I'm sure they would send a printed version free, but as you're in the UK I'm not sure about that. Go to:

See also: The latter is MOSTLY targetted to SolidWorks 3D MCAD users, but it has some other sources of information as well. And even the SolidWorks-centric information may give you some good clues about good design practices.

Mark 'Sporky' Stapleton Watermark Design, LLC