From a quick search I count at least five meats in the past ten years...
2024-04-03 msg:43024.1 Manchester Mini Meat
2020-01-20 msg:42105.1 Leeds mini-meat, Saturday 20ndsth Jan
2017-06-24 msg:41942.1 June Meat
2014-12-20 msg:41295.1 Saturday 20th December
2014-07-19 msg:41092.1 Teh Northern Meat - Summer 2014
None of those combine "London" and "well-attended", but I have photographic evidence of three that do, in Sep 2013, Sep 2010 and Dec 2008.