You will need to factor in time to go thru immigration upon arrival and to get through security upon your return. That said you have time to go into London and see something. You could also head to Windsor. The time it takes to get through immigration depends upon your time of arrival and your passport status.

If you can afford a mincab or car service do that, otherwise take the Piccadilly line in. If you are considering taking the Heathrow Express normally I would just book a minicab/car service when traveling with someone. While the Piccadilly line and the heathrow express can be delayed it may make your arrival time at LHR easier to predict. What time you decide to head back will depend upon your own personal comfort level and which mode of transport you decide upon.

See these threads for ideas:
http://www.flyertalk.com/forum/u-k-i...over-kids.html
http://www.flyertalk.com/forum/u-k-i...ation-lhr.html
http://www.flyertalk.com/forum/u-k-i...r-layover.html
http://www.flyertalk.com/forum/u-k-i...yover-lhr.html
http://www.flyertalk.com/forum/u-k-i...few-hours.html