I recommending calling the number listed on the homepage and asking the operator to speak to the front desk - then tell the front desk you want to raise a complaint and would like the appropriate email address.

Alternatively, ask the operator to speak directly to the duty manager, who should hopefully give you their own email address. This sometimes doesn't work as the operator will tell you they're not available.

If you do speak to someone, get their name and reference it in your mail - this may elicit a quicker response.