Room thermostat replacement

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I'm wanting to replace a vintage (80's?) sunvic tlx room stat as it is rather tired and dated .
I popped the cover off to look at the connections and this is where I'm a bit confused .
I was expecting a three wire and earth install . What I have is red and blue connected , an isolated (badly) yellow wire and an unsleeved earth that I presume goes to the back box . Is this going to be a nightmare to replace ?
System functions well at present if a little old fashioned . Help !

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Thanks Ricky , guess I was getting hung up on coloured wires . Most stats seem to be 2 wire now so should be ok if I can identify switch live and live !