Somebody said this on TV today and I haven't heard this since I was very young
It's a Yorkshire slang expression...
Question If I was about to make a "Mash up" what would I be doing?
Clue Everyday activity (especially British)

Sounds like you are about to 'make a brew' (Lancashire speak) or 'brew up' (more southern). Be careful not to let the leaves mash for too long or the tea will be stewed.

I've just looked up the word in the New Oxford Dictionary and they list it there as being northern English usage in general. Certainly I remember my mother-in-law (born in Carlisle) using it - "Let the tea mash" she used to say - though, oddly, not my mother who was born in Wallasey in Cheshire before moving to Carlisle in the 1940s. A Scottish friend I've just asked doesn't know that usage either, so it may just be the north of England as such.

Correct you 2 and it brought a faint memory to mind may not have anything to do with it ...I recall somebody putting tea into a basket (over a teapot) then pouring boiling water over it then they would mash the soggy tea leaves...I Imagine the tea would be very weak

Seems to make sense..Perhaps it was done during rationing in the War? Hence the expression?

Me? I prefer a workers strength brew with Milk and sugar.