Hi Rico,
Congratulations to your nice little trailer which is definitely worth to be kept in a road-worthy condition. Born in East Germany myself I grew up with these trailers (and other IFA vehicles) and I'm always surprised to see how widely they were spread over the western world.
From your photos, I suspect it's most likely a HP 400, maybe a HP 350. The three-digit figure gives the max. gross weight in kilograms. The labels you took photos of won't help here as they belong to the towing clutch. There should be another one, either at the front wall or at one of the drawbar rods, indicating the model name and more details.
By the way, there is quite an active club for East German vehicles in the UK: http://www.ifaclub.co.uk/ They will be excited to get to know another owner of an IFA vehicle, and maybe to welcome a new member. And I'm quite sure they can help you identifying the exact model. And when you have to get it back on the road or to pass M.O.T., they may have some good advice for you. Perhaps they can even tell you more about the importer and how these trailers came to the UK. But be aware, depending on how much petrol circulates through your venes, it might be like a virus....
I'm not an expert in these little ones as I'm owning an entirely different HP 500. What I can tell you is that wearing or vulnerable parts such as wheel bearings and rear lamps are still available from today's production. It might be a good idea to put some aside for just to have them in case of......
Wishing you lots of joy and useful mileage with your HP. Don't hesitate to contact me for translations (as long as it isn't for full manuals), I'll gladly be at help.
Best regards
Dirk