Definitely before in case issues show up requiring further investigation. Exchange of contracts is a commitment to complete on a specific date. Everything has to be in place before exchange can take place including the finances, check on title and your buildings insurance. btw as your vendors are now buying themselves they may want to wait for a simoultaneous exchange up the chain unless they have somewhere to store/live short term if completion dates don't coincide.
Before!!! Once you exchange you are committed to buy..if something comes up in the searches you may want to pull out. If your solicitor is not returning your calls I'd seriously consider getting rid of them, if you don't want to, turn up on their doorstep and insist that they talk to you. You are paying them to provide a service!