As you want to at least get the feel of the market before you put your house up for sale, surely you should contact all and every estate agent around you. Lots of people still opt for a sole-agent contract as the commisions are lower. So you might easily be missing out on your perfect house by only dealing with Gregory's.
While looking for the house you want to buy, you'll get a feel for the efficiency of the agents you are dealing with and will be able to make a much better assessment of who you want to sell through. And you should get a valuation along with commission and terms and conditions details from at least three estate agents anyway, so you can decide whether what the best balance of efficient agent/good t&c you can get, and this won't necessarily be the same estate agent that you eventually buy through. Why would you ever hope to do the whole deal through just one company.
And personally, I would let Sherlocks know exactly why I am not interested in either buying or selling through them unless or until the can get their act together. The UK housing market is so slow at the moment why would they want to risk losing potential properties to sell and lose a potential buyer just because one of their employees isn't up to the mark. I wouldn't try to email it, I would phone, ask to speak to the manager, and just ask him to remove my details for the time being as the contact you have been allocated is not offering you a satisfactory service. So many calls in one day or weekend is harrassment, and not to contact you at all is neglect; even if there are no suitable properties, he could send a brief fortnightly or monthly e-mail to that effect.