If you have a valid US Visa than you don't need UK transit visa.  In your case 5 years multiple visa should work.  Below is from ukvisa.gov.  Hope it helps.

Holders of certain documents are, regardless of nationality, exempt from the requirement to hold a Direct Airside Transit Visa when transiting the UK.

A transit passenger is not required to hold a transit visa if he holds or a person with whom he arrives in the United Kingdom holds on his behalf:

1. A valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel via the United Kingdom as part of a journey from another country or territory to the country in respect of which the visa is held;
2. A valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel via the United Kingdom as part of a journey from the country in respect of which the visa is held to another country or territory;
3. A valid airline ticket for travel via the United Kingdom as part of a journey from Australia, Canada, New Zealand or the United States of America to another country or territory, provided that the transit passenger does not seek to transit the United Kingdom on a date more than six months from the date on which he last entered Australia, Canada, New Zealand or the United States of America with a valid visa for entry to that country;

Jonny,

How would you intepret third sentence (six month requirement) and how does it apply with a B2 holders who is already here for 6 months and is on approved extended stay of another 4-5 months?

3) A valid airline ticket for travel via the United Kingdom as part of a journey from Australia, Canada, New Zealand or the United States of America to another country or territory, provided that the transit passenger does not seek to transit the United Kingdom on a date more than six months from the date on which he last entered Australia, Canada, New Zealand or the United States of America with a valid visa for entry to that country;