So if you have ‘done’ all the big things/been to the “must see” museums etc then what?
1) What (more) unusual sights/museums/buildings should I not miss?
I have already been to Highgate Cemetery (awesome btw).
2) I’m looking for a good & fun restaurant (not posh) – no more than £50 a head for a main + beer.
I have already been to Archipelago/Belgos/Momo (loved them all).

Dennis Sever's House -- pay attention to opening days/times as it is only open Sundays and Mondays at specific times
Benjamin Franklin House
Freud Museum
The canal walk between Camden and Little Venice
Also look into the various London Walks for day tours and evening pub crawls
For a fun pub you might like to try Ye Old Axe, it's on the Hackney rd in Shoreditch. Make sure you have lots of £1 coins ready.

I´d say a curry in Brick Lane area, I am from Denmark but always make sure I fly via London when going travelling, with time enough to leave the airport and pop by East London for a curry and a beer(bought in the next door liquer shop, since they don´t serve beer).And if possibly, Petty Coat Lane Marked too...
Reminder: It's London Open House this weekend (Sep 15-16); often this is the only time of year when you can visit some of the more unusual architectural sights around London.
