Welcome eeyore, its nice to see a fellow uk brewer and budding BIABer too. There's a few of us UK guys on here and there seems to be more joining so things are lookng good for the UK BIAB scene.
As to urns, 30litre seems to be the max in the uk this retailer
http://www.leylandhomebrew.com/ stock them i think or try this guy
http://themaltmiller.co.uk/index.php?_a ... ductId=192 its just a 50l pot I'm aftraid ( but very good value for the uk). If your not up to a conversion yourself give the guy a ring and he may be able to sort it for you. The backer elements he mentions are good as well, much better than any salvaged kettle element.
I've been thinking about my pot and how I heat it as I've got to get out of the house really, I've considered electric but boil time is an issue and I'm looking at a 25000btu gas burner.
EionMag is a guy on here (Ireland based) has a large urn setup shown here
http://biabrewer.info/viewtopic.php?f=24&t=306 it cost abit though

he might be the one for a bit of advice. You could PM him, I'm sure he wouldn't mind.
Where abouts are you? As an Electrical Engineer I should be able to help you out with a DIY build..
Sorry for the bit of ramble, hope this helps.
Y
Why is everyone talking about
"Cheese"