We visited Trebar yesterday with a moderately disabled guest. We booked a mobility scooter just before leaving (recommended) and it was ready for us on our arrival. Our guest was given an overview, a short test drive and then we were all off. The scooter was quite a powerful one so managed this valley garden without any mishaps and the route was well marked and very enjoyable taking in all the sights.
I can recommend the Trebar's restaurant with its locally sourced food (and in particular their sausage bap), although I believe the beach cafe is also very good (although not accessible by wheelchair or scooter unfortunately).
I think all Cornish gardens are best viewed in the Spring to enjoy the Camelias, Rhodedendrums and Magnolias, but Trebar has some lovely waterfall/pool features and, of course, the Helford sits at the bottom of the valley on a nice little beach.
Well worth a visit and dog friendly too (so long as they're on a lead).
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