If timber cladding is found on the walls (and balcony - single means of escape route) of a low building, then I understand that it must meet a rating of euroclass B. There are plenty of companies who sell fire-retardant solutions to paint the timber. Do these tend to be reliable? And do they need specialist knowledge of how to apply them - e.g. to achieve desired depth of impregnation?

It depends if they have been tested for the external application or not and who will certify the completed system. Certain products will only be sold to suitable installers - best to ask these people who have a lot of experience and tech support facilities -  https://envirograf.com/product/benefits-of-intumescent-paint-for-wood/

Thanks very much for your help.