Is it an Interpet Delta Therm by any chance? In which case yes it is normal. I've owned several of this brand over the years and every single one has had condensation. Others have reported the same. Interpet's website says it is due to humidity in the factory (which of course is then sealed inside the tube) and is perfectly normal, and no cause for concern. Not sure about other brands though.
Hi I had them replaced and so happen to be the same make you said Delta Therm they were different to the ones I took back so I hope I don't see CONDENSATION in them. I have a 2 year warranty with them and they will go back if that is the case. Because when they get bad you could have problem with your Eclectic Tripping out and I been told could cause a house fire. Not worth the hassle. Thank you for your reply.
It's certainly normal for Delta Therm and quite possibly for other makes too. I think my old Aquael heater had it too although it was hard to see. It's certainly never caused a problem for me and I can't see it causing a fire (it's underwater!) or tripping your electrics, especially with UK fused plugs. If anything it will blow the fuse in the plug long before it trips your electrics, but even that is unlikely. It's just humidity from the factory which is sealed inside the tube, and not a sign of leakage. It made me a bit nervous the first time I saw it, but I've since learned it's nothing to worry about.