ah thank you Rudeofus, that's a bit of information (about the chromium bleach) that I hadn't properly understood.
Reviewing the negatives this morning, there is a marginal improvement. Worth the effort to understand the process, but not significant enough to make much difference to a scan or print.
I think next time I need to order some bits and pieces I'll include some chromium intensifier and have another go. I'd still be interested to know more about the Speedibrews intensifiers.
It's not just Fotospeed by the way who are able to supply Bichromate. You can buy by the pound on eBay.