with Germany having more widespread snow than usual at the end of April, given the usual advice of O bis O (October to Easter) regarding winter car tyres, if I drive my UK car in parts of Germany which get a freak snowfall can I get in trouble for having normal not-winter tyres?
can I get in trouble for having normal not-winter tyres?
Yes, in serious trouble. At a normal police check it is "just" 60-80 Euro - maybe double for non-domestic drivers because they do not get penalty points. In case of accident you can become 100% guilty / liable for the accident (independent from really happened) as only driver without winter tyres and you can lose insurance coverage partly or fully. In worst case it is possible that you get filed for a crime - depending on accident situation.
Never consider it because you may play with lives or health of other people.
Please all, forget O bis O - this is decades ago.
One source for this topic in German language:
https://www.adac.de/rund-ums-fahrzeug/ausstattung-technik-zubehoer/reifen/sicherheit/winterreifenpflicht-deutschland/
Thread from a few days ago:
https://community.ricksteves.com/travel-forum/germany/winter-weather-in-april-mid-and-southern-germany