I had to replace/repair the inner tubes on my buggy and I visited Hadron Cycles on the corner of Hornsey Road and Seven Sisters. ( www.hadroncycles.com) They were great! They repaired my inner tube for free and ordered more inner tubes of the right size.
They also supply trailers for pulling kids behind your bike (which a paramedic told me was the safest way to transport a child on a bike, as they have rollbars and don't tip if the bike tips) and will attach them to your bike. They even repair bikes, apparently.
Basically, I walk past a lesser cycle shop for the superior service at this place. :o I'm sick of being condescended to by people who think repairing an inner tube is beyond me when in fact it's finding the time to do so while looking after a newly-mobile 6-month old that's beyond me!