Basil, sorry I haven't replied - completely slipped my mind. No, it doesn't always do it. but, this morning did it on another computer. Funnily enough another website did it on the same computer last night but was fine this morning.
I'm writing this on IE which shows the site fine, and, this is the version of Firefox
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.0.13) Gecko/2009073022 Firefox/3.0.13 (.NET CLR 3.5.30729)
thanks
JP