I happened to walk past today - 10am - 4pm, closed on Thursdays and Sundays.
Looked like they had some good flowering plants and seemed cheaper than places like homebase.
It's worth a visit but I'm afraid I find it a bit pricey - though I think it supports a good cause. Really recommend the garden centres at Enfield/Crew's Hill.
I think it's a chap called Ray who is in charge there, and when I went in looking for Geraniums a few months ago, he did me a deal of £2 a pot for good big mature plants (I got 6)
If you want more than one thing, I think it's worth a bit of a haggle... they'd rather have some money than not I would think. And it it a very good cause and a lovely place to pop into!
Thanks for the responses.
I decided to head back that day in the hope it was open. I did, it was, I'm happy!