While there are review sites available for people to give their opinions of hotels, holiday firms, retailers and many other thing, there simply don't seem to be any for reviewing solicitors. (I suspect that sites like TrustPilot won't review solicitors because of the possible legal action that might occur if they published an unfavourable report of a solicitor simply from someone who'd lost a legal case despite excellent representation).
However Debt Guard is part of Irwin Mitchell, which is the biggest 'full service' law firm in the country and highly respected. So they can be relied upon to provide a professional service.
Even so, there's nothing that any debt-chasing solicitor can do which you can't do yourself. If you're trying to get someone to pay up, consider using H M Courts & Tribunals Service's Money Claim Online service to initiate court action through the Small Claims Procedure:
https://www.moneyclaim.gov.uk/web/mcol/welcome