For what it's worth I def think you get what you pay for, and I think that some agents are better for the sub £500K market, others for the £500K to £2M & then the Hamptons, Savills, Knight Frank etc for the £2M+ where the highest stamp duty rate kicks in.
I've come across most of them & I would agree with Jonathan of Hamptons, we had nightmares with a purchase & sale & it if wasn't for the agents, it would have fallen through on both occasions. The agents made it happen when the solicitors, surveyors had missed things, not communicated etc.
It also helps to have a good agent for your onward purchase, again our agents saved the day when the agents for the house that we were buying were less than honest about exchange & completion dates & we came close to ending up with no where to move to a week before Christmas with a 2.5 year old.
When our sale & purchase finally went though, I was very happy to pay the fee, and hand deliver champagne & flowers to the team.
Good luck & PM me if you would like recommendations