Well, I've had a quote of £180 provided I supply the new parts myself. 
Plumbers can't be short of a bob or two. 
You’ve got to decide if it’s within your capabilities. If it’s not, be honest with yourself and just pay a tradesman to sort it for you. You’ll soon forget about the money and be glad that you personally didn’t have the hassle of repairing it.
Honeywell 3 port motorised mid valves are around £128. That would be the finished job cost if you do it yourself as no sundries are required (drain down would require Fernox upon refill of system). I’ve done enough plumbing over the years to have enough tools, pipe and connections to get myself out of the shit should something go wrong on a job like yours, but if you haven’t it could end up messy.
You need to be confident enough to drain down or isolate that area of the system, which means understanding how your system works.
The valve nuts need to come off regardless, and your valve has spanner flats next to each port nut which can minimise pipe twist so that’s a bonus.
You require basic electrical knowledge to swap the valve wires over. You require enough knowledge to fill and bleed the system once you have replaced the motorised valve.
It’s a relatively straightforward job to some but if any of the above worries you, just sit back and let a plumber do it.
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