To Andy, a blank chip is interesting, but I'm guessing you need rather more than a GS911 to copy & encode a new one.
To Matt, the latest Tile Mate doesn't have a replaceable battery, so I'm not sure which variant that case is aimed it, but I guess it's an older one.
Amazon has the cheapie Mate for £11.99 this afternoon (spesh offer), so I bought one with no replacement battery, and sod the environment.
I'll have a play and let you know if it was 2 pints of Aspalls wasted.