One and a half hours should be fine. 19:30 on a weekend is not Heathrow's busiest time so if you do find yourself rushing at least you won't be struggling against tides of people!
I've not done T2 to T3 so can't comment on specifics - I presume it's similar to T1 to T3 which is
5-10 min walk to connection centre
10 mins to check in (if you have to)
5-10 mins wait for bus
5-10 mins on bus
10 mins to gate
Why not post something on the LHR board?
edited because I can't spell!!!
[This message has been edited by matthewuk (edited 07-13-2001).]