Thanks for the comments guys!
Tim, yes it will have a back rest eventually. I've sent drawings off to companies for quotes but sometimes I just feel like I'm bashing my head against a brick wall!
The only company to get back to me promptly was marine fabrications (The one Hugh Jardon used) with a quote of £137 + £25 P&P. Another company informed me yesterday that they would be charging £283+VAT for the same job
HOw they can justify charging that much I really don't know!
More updates soon I hope.