Tide times and measurements

When you look up the local tide times, you get the times of high and low tide for the day, but you also get a measurement in metres to go with each time. I've always wondered what specifically the measurement is. I know its how deep the sea will be at high and low tide, but in relation to what?

Example, low tide in Redcar today will apparently be 0.9m. If I stand on the beach below the high tide line, I wont be in 0.9m of sea, I'll be on damp sand. If I stand in the sea just below the low tide line, I'll be ankle deep.

Does anyone know what the measurement is relative to?

 

All heights (land and sea levels) are linked to sea levels at Newlyn, Cornwall http://www.seazone.com/uploads/refzone-Datum Reference Paper.pdf and http://www.ntslf.org/tides/datum

 

Excellent article there @Scrungee , I've read all of it and understood about half it, which isn't bad going for me

I understand the bit that answers my question though, I think.