What were they doing on the motorway in episode 1?

I've thought about this a lot myself, and I've got it down to two possibilities:

A) Guillam is checking to make sure they're not being followed, or

B) The notorious unreliability of 1970s British cars has forced him to stop and make an on-the-fly engine repair.

I've watched this series several times, and I think that I've come down on possibility B as being slightly more likely. Anyone know how reliable a 1978 Morgan 4/4 was?

Peter Guillam was in charge of Scalphunters and one of its operatives, Ricki Tarr, was on the "wanted list" having gone missing for three months somewhere after his Portuguese mission. There was the suspicion that he might have been "turned" by the Russians, (Percy Alleline's theory) and he had important information concerning a potential Russian mole in the Circus. Only Guillam and Sir Oliver Lacon knew of his presence in England.

It follows that Peter Guillam was being very careful to make sure that he was not being tailed - hence the high speed journey in his sports car, the frequent glances over his shoulder and finally the stop in the lay-by to discover if he was being followed. Eventually, after raising the bonnet of the sports car and satisfied himself that the lorry had pulled over for entirely innocent reasons, they set off again for Sir Oliver's mansion that he referred to as his country Camelot and you may remember was likened less generously by others as Dracula's blood bank! (Good line).

From the very first time I watched it, I was convinced that he was checking for anyone tailing them. They were on an ULTRA top-secret mission, and the entire Circus could have been turned out looking for Tarr or anybody trying to contact him.

I thought the only reason he popped the hood ("bonnet"?) was to make their delay appear "authentic". Like when a spy bends over to tie their shoe so they can look behind them without being too obvious.