Thanks, I've managed to do this now.
Although there seems to be a problem with alignment (in Firefox) where there are help icons - these get spaced to the right of the .UDT_Caption width.

screenshot here:
http://www.sypensions.org.uk/webCompo...

I am using Form and List 5.1.1 (as it's the latest version I can use with our sytem - DNN 5.1.1 .net v3.5 sql server 2000)

Not sure if there is a way to fix this without upgrading?

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for anyone else wanting to use css...
I called the css via the header properties in settings

<LINK href="http:www./folder/form.css" rel="stylesheet" type="text/css">

and used this css:

.UDT_Caption
{
    padding-top:5px;
    padding-bottom: 7px;
    padding-left:7px;
    width: 300px;
    float: left;
    position: relative;
}