Hi Richard - I think it's about 3 millimetres. That's exactly what he was going to do, but we have parted company so will probably have to do it myself. There is tile board behind it all, and then some kind of waterproof adhesive. Then grout. I think that's all. Would you tile right up to and flush with the trim, or leave a gap for some grout. I'm really unsure of the correct way to do this. I know there needs to be a slight slope on the bottom tile. Would you use Silicon anywhere? Any help would be great!