Dave L Wrote:
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> So what's the explanation?....

The line you were standing on was the Airy meridian. GPS (and Autoroute and anything else satellite-based) refers to the Terrestrial Reference Frame (see [www.gps.gov.uk]); the difference is currently a tad over 102m.

>
> Well for a start, after a bit of research, it
> appears the line marked on the ground is the older
> 'Airy' line, whereas the one used on maps is
> apparently another one, the 'Bradley' line,

The Bradley line is older than the Airy line! The Bradley meridian is used on OS maps (OS predated Airy).

> Any suggestions?

Only that any positioning system must refer to a reference frame and it is rather important to know what that frame is if one wishes to avoid confusion. (I know, stating the obvious, but...)

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Stephen