Share ch boiler with back boiler

[drongo]

I plan to replace my old Ideal CH gas boiler. It runs only the radiators. I get my hot water from a gas-fired Rayburn cooker. I want to keep the Rayburn, but it's a very inefficient way of getting hot water and I tend to use it only in the winter.

Ideally, I would like to have a system with pipes & valves set up so I can get my hot water from either the CH boiler or the Rayburn. I was told by a BG gas engineer that this used to be quite common but they're not allowed to do it in these days of Health & Safety regs.

Is there some way it can be done? Is it illegal?