We took advice to pay the extra on top of the cast...

We took advice to pay the extra on top of the castle price and see the gardens and it was only worth it if you join a tour of the garden. We had the excellent Jackie as our guide and she made it an enjoyable and informative tour. Her enthusiasm was infectious and we then went on to the poison garden. Although interesting our guide wasn't as good as he could have been and missed a few plants out like the deadly nightshade.
Overall it is the price that would stop us coming again, but Jackie was at pains to point out that the profits are ploughed (pun) back in to the gardens and the duchess charitiable work with the community.