It looks like I will have a long connection at Heathrow on a flight from Nairobi to Ottawa in March - 5:45 arrival at T4, and 14:00 departure from T3.
Do you know whether you're going to be able to get your bags through-checked? Or will you have to collect them at LHR, schlep them over to T3 and check them in there (probably after putting them into left luggage, and then retrieving them again before check-in)? That will have an important effect on how much time you really have for a trip into town.