Advice please?

Looking to fit a wooden window sill to an already slate tiled window sill area. Ideally looking to bond/adhere in some way.

Suggestions for best adhesive?

Does it need to be a waterproof seal? I'd use either adhesive foam (expanding foam that doesn't expand) or regular silicone sealant. Either will stick like the proverbial.

In a bathroom, so yes, waterproof.

Liking the foam idea.

Any brand recommendations?

I use a lot of the plasterboard adhesive foam by soudal, or window frame fixing foam. Both at screwfix. However, I'm not sure the foams are waterproof. You'd have to finish with silicone anyway.

I wouldn't use a sanitary silicone for bonding to anything vaguely porus. But an LMN silicone is superb as an adhesive and sticks to everything (as long as it isn't wet)

Sikaflex or any pu glues
G

CT1 sticks almost anything to everything