I go to a place on Tower Bridge Road, just past Bermondsey Sqaure on the opposite side of the street. Can't remember what it's called but it's £5 for heel repairs and they always do a really good job, pretty cheap to get new soles too.
Also try Cobblers Nest on Redcross Way, the chap there is very professional and friendly, does all sorts of repairs and very reasonably priced. The one on Borough High Street is a total rip-off - as you have unfortunately found out - sadly they are just exploiting their position on a busy street. They also charge around four times the price for keys cut as you would pay at Worralls, the proper locksmith, on Great Suffolk Street.
I second the recommendation for The Cobbler's Nest - Eric's very friendly, his prices are reasonable and his repairs are high-quality. He's also - more than once - taken on repairs which other cobblers including Kehoe's on Borough High Street have declared impossible.
I can also recommend the Cobblers Nest. What we used to call a proper shoe mender and not just a 'heel bar'. Excellent quality, although you do have to be a bit flexible sometimes as to when more complicated jobs will be ready. He replaced zips in my boots which no other repairer around would touch. You do need to leave a deposit when you take shoes in, so be prepared. But all is worth it.
So, on one of my rare days in the City, I went out of my way to take my favourite boots to the Cobblers' Nest.
Not open at 0825. No opening hours displayed, as far as I could see, so my boots are in Timpsons in the City, costing a fortune. Does anyone know whether the Cobbler's Nest opens Saturdays?
I'd love to support local businesses, but it's so hard to do. It's another example of businesses and services not opening at times that locals with jobs can actually get to them (eg the health food shop and the GPs on Lower Marsh).