"Phrases like "slip into the accent" really piss me off."

Robbie, sorry it offends you but I can't really think of a better terminology.

My us of "slip into" is because it is not something I intentionally do. I might sometimes be in conversation with someone and later somebody else points out that I was speaking with a different accent, which I hadn't been aware of.

Having been brought up in Shropshire I was exposed to yamyams of Wolverhampton. I never had such an accent but when I speak to someone from there I find I can't stop doing it for a while afterwards... and I certainly don't do it on purpose.

It's also a point to note that we often imitate (consciously or not) sounds around us and find ourselves picking up accents through interaction. many of my English language students here in Poland learn English from Polish speakers and have a Polish accent, others might learn from Americans and have an American one... then they go to Ireland and come back with an Irish one. But I don't know if they keep that accent when they speak Polish.