entering twice with a visitor visa

[samforward]

My girlfriend was approved for her visitor visa and will be coming for a month this Christmas. She now has to fly abroad to a third country the same day she arrives in the UK for one week before returning here. She gets into the UK in Gatwick and leaves 5 hours later in Heathrow.

My question is does she have to book another transit visa to travel between airports the first time around? Or can she use her visitor visa when moving between airports and again when she returns for three weeks.

Thanks