I have been collecting data on birds in the garden for BTO Garden BirdWatch since 1995 and I see from the records that the first time I saw a redpoll here was in 2011. Interestingly the BTO still list it as a scarcer species so no long term data are available. I did find this BTO plot showing a huge increase in numbers in gardens in recent years.
Despite this the lesser redpoll is on the Red List and is much scarcer than it was years ago.
You can read about identification of lesser redpolls, common (mealy) redpolls, and arctic redpolls here. Read more about lesser redpolls here. Listen to Sir David Attenborough's BBC Radio 4 Tweet of the Day on the lesser redpoll here.