You must be doing it wrong, purchasing flights is not difficult so I'm at a loss as to why you are having so much trouble.
1) Go to Britishairways.co.uk
2) Select 'Multi City/Stopover' under the destination drop down box
3) Enter Manchester to Boston on 4th August
4) Enter New York (all) to Manchester on 18th August
5) Click get flights
Select the flights you want. Get price of £779 per person depending on flight choice. This includes the domestic leg to Heathrow and back and the transatlantic portions. There is a wide range of available flights to pick, too.