Welcome to UKCS - I expect you'll find it as addictive/useful as the rest of us. As to your question - no you don't have to just use a Sunncamp EHU any make will do as long as it's got a RCD fitted and is 'safe'. Do a search for loads more info on EHUs they're a regular topic.
Buy one from a reputable camping/caravan shop either the Sunncamp or similar. Expect to pay around £50 for a 3 socket version. Towsure are good on price but shop around. Don't be tempted by a cheap version from Ebay. Some sites may ask to inspect your EHU and will ban it's use if it's not up to scratch. If it's not a proper job you also run the risk of leaping around like a madman with your hair on fire as 240 volts make their way to earth via your good self.
------------- HelenD
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We must be experts - we are out standing in our chosen fields!
Most look very similar to the sunncamp one, so if it looks the same, and its from a reputable shop, it should be fine. All too often I have seen tents on sites, and offers on e-bay that are no more than a 4 way extention with a 16A plug on the end, these are NOT safe.
Just one point to mention, look out for the orientation of the plug sockets. You can get three socket EHUs with horizontal or vertical socket orientation. The best unit to get is where the leads exit from the side rather than the lead trailing over the next socket. This is because sometimes dependant upon the plug you might find it difficult to use all three sockets at once
I'm looking for one of the horizontal ones but the cheapest I've seen one so far is £85.
I asked the question as to any problems with accessing the middle plug and only one person replied saying that they could use theirs no problems.
There's a man who has a camping shop replied to my question and said that there is a new triangular one being released for sale in April and will post a picture of it when he gets one in.
I have to say that it is a solid, quality peice of kit and better quality than the sunncamp one that I looked at in a camping shop. It has 20m cable, horizontal sockets rcd, mcb a carry case and £5 cheaper inc delivery. Highly recommended. Obviously won't know if it works until I use it on a site for the first time
(And I'm not affiliated in any way with the seller)
------------- I'd rather have it and not need it, than need it and not have it.