Wickenby
I have to agree with the threads the posters that have listed Wickenby, depending onwhere in Lincs you re based. Judging by Waddington being close id say Wickenby may be the best bet, plus Ive flown into Wickenby a number of times (first Solo land away was there) and always found it friendly, and with good facilities.
Whilst i agree that Long runways can be of benfit when learning, I learnt ona 600 m grass strip and this, in my opinion, has been more beneficial as since passing my test im happy to fly into anywhere. When speaking to people who learnt with big long tarmac runways they often feel the need to do farm strip conversion after passing, or just avoid places with short grass runways which is a shame.
Whilst Wickenby is not a short grass runway, I believe it flys into some(including the one I learnt at) during the PPL which should help with confidence.
Although as a glider pilot I would imagine you would be ok with this anyway.
Good Luck with where ever you choose to learn.