Hi Colin, John & Kevin,
Many thanks for your replies,
I did speak to safety devices but they were not very helpful, didn't have details of history of a cage for a herald and at the moment too busy to consider taking order for a bespoke order. That's when (on recommendation from a classic friendly garage) I went to the local firm, does work for Custom Cages.
I'll make some enquiries with the people you suggest, and have a perusal of the blue book online.
Cheers Pete