Thanks for the quick replies.
As it's obvious I am speaking from personal experience here i'll give some further information to get your opinions if you don't mind.
I, a planning consultant/designer, have been approached by a new building system supplier to become a technical director (designing details, etc) in return for a small shareholding in this company.
The building system name itself would not be identified in the my planning drawings but the type of building system may be if the client was acceptable to this type of build. E.g If I was a director in a specific timber frame supplier, and specified a timber frame on the drawings. (P.S Its not a timber frame company).
Independant reputation as you say is highly important and I would aim to provide an unbiased advice by giving the pro's and con's of all systems including the system i'd be involved be.
The client would still have an option to go with compeditive manufacturers systems at tender stage and choice will no doubt be primarly based on price.
So to sum up, once I declare all interests to clients prior to advising them, and possibly even declaring it on my website you would see no legal implications from compeditive manufacturers of this or other building systems. Is that correct?