Problems in qualifying left the Banbury-based RAW Motorsport team with a race against time, as there was only 90 minutes between the end of qualifying and the first race.
McAlpine said: “When I left the assembly area there was a red light on for the water temperature, then it went off. So I felt I was starting to settle and ready to go for a time and it just stopped.”
From the start the field soon spread out but McAlpine made rapid progress and was into fifth by the end of the opening lap.
The four cars ahead continued to pull away over the rest of the race, while McAlpine started to come under pressure from David Tagg by the end of lap seven.
“He got me going into the Shell Hairpin but didn’t get away as he kept braking earlier than me,” he added.
But McAlpine’s decision to resist the urge to rechallenge was rewarded only a lap later when Tagg spun off and handed back fifth place, which was retained over the final three laps.
In the second race, a group made an early break again and McAlpine found himself chasing from sixth place by the end of the first lap.
McAlpine said: “It felt more consistent and I was up to pace much quicker than in the first race. The car was good and over the weekend it gave me a lot of fun and valuable seat time.
“Although I was on my own I learned how the car reacts and could tell the grip was much better too, a 50 per cent improvement.”
Having been lapped by the leaders, McAlpine approached Lodge Corner for the final time and headed for sixth place but fifth placed Mark Williams had crashed out and sixth suddenly became fifth again.
The next round of the Championship is at Snetterton on Saturday, May 12.