To update - not having much luck with this.
I ended up applying in June. It arrived at the headquarters a week later (I tracked it online). Stupidly, I forgot to include my passport photos, but I received an email from someone there mid July (personal email address but asked for the passport photos for the police clearance certificate, so I assume not a scam), and said I could send a digital copy. Since then, I've had no reply. I've emailed that person again, emailed the police department, filled out the contact form on their website multiple times (which says you will receive a reply within 24 hours).... nothing. I don't want to apply again as I already had to go to Bristol to get two sets of fingerprints (about £150 for the pleasure). My HR department are quite rightly chasing me up on this frequently, and it's difficult trying to explain the reality of Thai time when their website says the certificate will be sent within 25 days!