If anyone's wondering how an agent can have such a reasonable average score, yet a ton of scathing reviews, it's probably because they offer a £10 M&S voucher for (their words) "a favourable 4 or 5 star review". Note how anyone who's written anything of note has said quite the opposite.
In short, D&H are at best inept and at worst deliberately negligent. Issues in a year's tenancy include:
Delaying our move-in by two weeks a few days before the due date, leaving me homeless;
Failing to fix both major and minor faults despite repeated emails, then charging us for the repairs at checkout;
Charging us £50 for paying rent a day late whilst we were waiting the mercurial Greg to get back to us on an updated tenancy agreement;
Refusal to reimburse us for getting a key cut for them to show prospective tenants around - a key that they didn't have because of their own uselessness;
Pathetic customer service throughout, they only seem to realise what email is when they need something.
Based on conversations I've had with a couple of friends, this is probably the worst lettings agency in London. Avoid like your life depends on it.