Letting agents, if in use, is one possible point of contact. Might be why the tenants have no contact details.
The tenancy agreement may have the landlord's current address for him.
The Land Registry may have his updated address on it if he thought to change it when he moved (possible he didn't though). You could download a copy of the title register from the Land Registry website to check.
As a last resort, I doubt you want to incur additional cost, you may be able to find a cheap people finder on the internet on a no find no fee basis.
Have you tried any sites like Facebook or BT phone directory, see if he comes up on any of them or anyone you might know who is associated with him, family etc...