Andy,
I also had my first Red Admiral of the year in the Doddington Hall area (today) ...... over 3 months since your earlier sighting.
I've checked this Lincs Butterfly News website and I couldn't find any other Red Admiral record for 2010, although members may not have bothered to enter a record of their having seen one; or I missed the web record.
I presume your sighting was of a one-off early immigrant, rather than an overwintering one, given the vicious winter we had......unless perhaps it had found a cosy nook in the Doddington Hall greenhouse complex.
Regards,
Freddy