Hi capetown2k - can you give us some more information about the type of ticket you hold already? Is it an award or a purchased fare?
If it's an award, maybe there isn't award availability for the MAN-LHR flight.
If it's a fare, it might be non-changeable, which would include adding on a leg.
If you do buy a separate ticket, your bags can be checked through but you may not be protected for a misconnect should that occur.
Can you give us more details and we'll try and help.