Many Thanks Friends
Today I had a good look around the machine, cleaned around the lid seal and fully rinsed the foam cover to the VAC inlet. Indeed, the suction at the vac-lead socket on the machine now feels as good as I can remember (I'll consider this more important that the guage reading, which could be theoretical !). Perhaps in the end the problem was/is down to my imagination - bearing in mind the importance of leaving the carpet reasonably dry at the end of the cleaning process. One quick job today left the carpet very dry !
Again, thanks for your contributions, it's good to know others are around to advise.
Pete (JS2)