An update to the thread, for anyone curious about doing Hamerton who is reliant on public transport, considering that I did this very thing yesterday (Tuesday 27th March)
It is perfectly possible, on one condition - you have a general level of personal fitness! We did the following:
1) Got a train to Peterborough
2) Caught a bus from the main depot over the road from Peterborough train station
3) This was the 46 bus, which runs between Peterborough and Huntingdon.
4) We got off the bus at Alconbury Weston, outside the Silver Hart pub.
5) This pub is on a junction with Hamerton Road, which after passing through Alconbury Weston continues for about 4 miles before reaching the village of Hamerton.
6) The road then turns sharply right and continues for about 200 yards, at which point you should see the entrance to the zoo.
The walk to the zoo is quiet and along a little country lane which - when we walked there - saw very little traffic whatsoever, to the level of perhaps one car per mile. There is also plenty of space beside the road for pedestrians if need be. We managed the walk in a shade under an hour.
Thus, as long as you are reasonably fit and healthy, don't let the walk put you off

it's well worth the effort, I reckon.