Nickbrands measurements are correct. Unfortunatly it has filled up with fallen leaves and branches at the moment. It can do with a clean out. Next time i am down i shall clear it out and take measurements for you.
Morfe (tip of the Hatlo hat!) has just had a go at enhancing my original rather dark and dreary photograph of the basin, with some success. Might give you a slightly better idea of it, 4W. I intend to get some better pictures there one day when it's bright, and I'll measure it properly then. Assuming I'm not chased away by the rabbit assasin, that is :-)