I crossed over the old Severn Bridge yesterday, and I noticed that the odd looking building is still there, just before you reach the bridge itself, going eastwards. I looks like an electricity sub-station to me !
Sorry mikey I've just seen this so apologies for not answering.
I looked as I'm going that way today and was going to see what it was so was looking up old thread to see how you described it. I'll have a look later and get back to you.