This rumour does the rounds every so often, then there's so much laughter at the prospect od Serco even getting past the first round of putting tenders into envelopes that it's a complete non starter.
The facts are that Serco Aersospace barely have enough Atcos to fullfill the pitifully few remaining insignificant contracts that they have, additionally it would take the agreement of all the HIAL atcos for Serco to transfer to them, something that will never happen.
It is fact that Marshall Aersospace have been closely involved in various aspects of instillation of facilities and the setting up of approach radar services at Inverness and that they will be closely involved in the "management" of ATC services there.
However, given their performance at another airport elsewhere in England (not EGSC but not far away!), they also have an awful lot to learn.