On Sunday I noticed two pfg with neck collars. After 30 mins careful obs and some digital images zoomed in to max I had the 3 letter codes. I emailed WWT and received copious notes (6 pages for one bird).
As this blog is from my phone I'll keep the notes brief.
Wintered 2002/3/4/5 Martin Mere, Norfolk 2012, Martin Mere 2013 and now at Stobswood 09/03/2014.
Recorded Scotland 2007/8/9, Norfolk 2012, Scotland 2013 and now Stobswood 09/03/2014
Thanks again to WWT for quick reply.