Further to the background info above: I have now requested personal engineer support through my ISP. The site was developed under the IP address http://213.171.221.104 This site is working well it's just when the domain name is used that we get the errors. He has given me a bit more information and has said  "When DotNetNuke was installed for best4hair.co.uk was it installed under the default website and not the website address itself?" then added to this "The site when accessed via the default website works fine - but when access via the entry for best4hair.co.uk does not show the menu.

There's not a lot of differences between the two sites in IIS - bar a few permissions differences.

I believe it could be down to the install itself - and possibly the install of the menu module - I presume this is installed as a separate DNN module.  It may be worth creating a completely fresh install of DNN for the best4hair.co.uk IIS entry - pointing the site to a different/test folder first of course."

Does anyone know if this is likely to be correct. If so what action should I take ?  (The site is skinned in a Solaro package)

I think I might have stumbled across a solution. I've been looking at some of the other forum posts and found a solution (hopefully permanent) whereby I have deleted a PortalAlias in the host settings and now it seems to find it by it's domain name and not just the IP address. Admittedly, it reverts to the 213.171.221.104 in the address bar but otherwise it's fine and without runtime errors. (Hence i've backed up with the new settings now.)