We are travelling from Heathrow to Beijing Capital Airport then onto Singapore.

If both flights are with the same Airline and on the same ticket ( there are exceptions to this...different airlines, but same airline alliance group for example, but generally must always be on same ticket) then there's no reason on earth why not. You should be given boarding passes for both flights at check in at Heathrow, and your bags automatically checked straight through to Singapore. All you are left to do is go through transfer procedures at BJ. Enjoy your trip