Thank you all for your encouragement and advice. I looked it all up on google, and learned a great deal - EG: I had no idea water tanks come in so many shapes and sizes. There was even a full detailed page with drawings on exactly HOW to do it!!
I rang 3 attic-tank-replacement companies and got prices from e350-500. To the e500 one I said "thats a bit pricey!"--"hang on", said he, and a few mins later came back and said "well it could be e390, if its straighforward". I then phoned a plumbing suppliers shop and got incredible helpful tips and advice from a guy called Kevin, ended up driving there yesterday, buying everything I might possibly need for e111. And got names of 3 plumbers from him, whom I can ring and ask them to do a nixer. I phoned one who will ring me back this coming week. So far so good. So thank you all. Sincerely hope it wont cost me e700, like one poster's mum!