Hi Kathy,
If you have all the details of where and when you saw it, then yes, it would still be useful to let them know even though it is retrospective from being 2010.
Also, even though you saw this bird at WWT Slimbridge and chances are, they've submitted records of this bird, if it were me, I'd still send the record just in case.
I'm sure that The Netherlands Institute of Ecology would be very interested to hear from you (maybe you could 'enclose' your photo of the bird with your email to them too.) The Institute says on the cr-birding website: " is extremely interested in observing their foraging behaviour during the winter, as well as their migration patterns throughout the rest of the year."
:t: