The Lincolnshire Bird Club

I will post some photos later but for now here is the text from a BB article

Racial identification and assessment in Britain: a report from the RIACT subcommittee
Chris Kehoe, on behalf of BBRC

Red-necked Grebe Podiceps grisegena holboellii,‘Holboell’s Red-necked Grebe’ This North American and northeast Asian taxon is on the British List based on a single specimen record which pre-dates BBRC; there are also at least five records for Iceland.
Biometric differences are diagnostic, with holboellii being distinctly larger than nominate grisegena; average bill and wing lengths fall largely beyond the maximum of grisegena. Biometric details are cur- rently required for acceptance. A further distinction is the bill pattern of breeding birds – holboellii typically has extensive yellow on the lower mandible – but this may be only an average character and may be partly age-related; more research is required. Images of birds in breeding plumage with exten- sive yellow on the lower mandible would be welcomed. (Ogilvie & Rose 2002; McGowan 2006)