>>> Out of the 70 odd Freeview channels advertised we can receive 29.
I've just checked the Freeview channel availability using the postcodes for Puckeridge Community Centre, Buntingford High Street and Wheathampstead Memorial Hall (all of which would seem to qualify as 'rural Hertfordshire'). Based upon those results, you should be able to receive between 88 and 93 SD Freeview channels (although there are a handful of SD channels which you actually need a Freeview HD TV, or set-top box, to receive because they use HD multiplexes).
So, without a doubt, YOUR AERIAL ISN'T UP TO THE JOB.
It has to be remembered that when the switchover to digital TV was first planned, it was expected that around a third of homes would have to replace their aerials. As things turned out, far fewer people actually needed to do so but it still shouldn't be unexpected to find an old aerial, designed for analogue TV, not much good for receiving Freeview. (It's not the fact that Freeview transmission are digital which makes the difference. It's the fact that they use different frequencies to the old analogue transmissions, meaning that the aerial's elements don't 'resonate' well with them).
You need something like this on your roof
https://www.maplin.co.uk/p/nikkai-tri-fold-outdoor-tv-aerial-a22hg
possibly (if you're in a
very poor reception area) with one of these as well:
https://groceries.asda.com/product/tv-accessories/philex-signal-booster/910002320209
I'm prepared to bet that, by spending a bit of time and money sorting your aerial out, you'll then be able to receive all available Freeview channels.