At last, Apple have resolved my wifi issues!
While I accept that all of my issues were likely caused by my ancient Netgear G router with ancient firmware, no large organisation should deliver software that is not backward compatible.
Issues that have been resolved:
O wifi disconnect when coming out of standby;
O poor performance / 'hanging' in applications such as Safari and Facebook.
If you have wifi issues, try IOS 6.1 and hopefully they will be resolved, as they were for me.