The Bristol used Zenith/Solex carbs. Zenith is part of Burlen Group, which also manufactures SU. And yes, still in business supplying the restoration trade. They can help with his carb-related issues:

https://zenithcarb.co.uk/shop

These are constant depression carburetors, meaning that venturi vacuum is maintained at a constant level by a diaphragm that expands or contracts the cavity above the jet in response to airflow. The fuel/air ratio is regulated by a tapered needle, which rises or falls in the jet with the diaphragm. Problems can be caused by modern fuels, because E85 has a lower stochiometric ratio than straight gasoline, requiring a change of needle taper to richen the mixture. So simply changing to non-ethanol fuel may help.