Mimi
Hi, I'm also in N15, have a 17 month old and am back at work full time at the moment but will be on maternity leave from just after New Year, second baby due at the end of January if you're still looking to meet up with people then. Whereabouts are you? We're near Chestnuts Park if that means anything to you.
There are baby swimming lessons at Tottenham Green pool on Philip Lane but it's quite hard to get booked on to them. They're £50+ a term which is a lot less than private lessons, but there are a couple of cheaper options:
There's a FREE session called Splash at Wednesday mornings at that pool for parents and under 4s. It's not lessons but it's songs and games and it's free, it's funded through one of the Council's Children's Centres in our area.
In Hackney an organisation called Greenwich Leisure puts on baby swimming lessons at several pools, nearest to N15 is Clissold Pool, just past Clissold Park a few minutes walk from Green Lanes (341 bus).
When I was on maternity leave there were some lovely baby groups - Baby and Me was Tuesday afternoons from 1 at Downhills Park Primary School (on Philip Lane at the West Green Road end), there was a group at Broadwater Farm Children's Centre Thursday afternoons, and there's a a toy library session/music thing at St Ann's library on Monday and Saturday mornings. dp usually takes our son on Saturdays.
There's also a singsong on Thursday mornings at Marcus Garvey Library (inside the Leisure Centre where the pool is) at 11 am on Thursdays - I never made it to that one but ds goes with his childminder and another little boy she looks after quite often. Although DS still doesn't sing at the time, he's shown that he enjoys it and often sings the words to himself later.
Hope some of this info is of use to you, do post if you find anything good to go to.