My current rear mech has almost worn through the cage and the jockey wheels barley have teeth left so I feel it’s time to buy a new one.

Im running 50/39/30 and 11-28 so i’ve read that id need 37T capacity derailuer.

The bike is a sirrus comp (2008) and has the stock Shimano Tiagra long cage. I can replace like for like for £25. Can anyone suggest a better (lighter/faster shifting) one for around the same price?

Got the same bike, i stuck an xt mech on and it works fine. 9sp.

The 37T range can be done with a medium cage Mountain mech, but you can run a Road Mech for a 28T sprocket.

Bottom line is any medium or long cage 9-speed mech will be fine.

For £25 you will be lucky to get a used XT, but possibly LX or SLX.
Not up to speed with Road Mech prices.

Have a look at the CRC sale?

Merry Christmas, PaulD