I used Farmkey. The office are rubbish, no idea what they're talking about, managed to book my appointments so they were on the exact days I had said I couldn't do, then kept forgetting to send written confirmation
The marking technician was great though, v professional with my pony who was shaking from head to foot, and v quick. And I guess that's what really matters.
Depends where in UK you are, there is another company doing freeze marking, name and details are on http://www.tracingequines.co.uk under Freeze Marking
i used them last month, not overly impressed,called my horse a thug, apologised later tho but even still, not a great deal of patience, think i need them to come back to, how long should it take to be visible, all 4 digits i mean?
If it is only within a month, then you will need to give it a few weeks longer at least. You didn't say which Company you used, but Farmkey are reknowned for having to re-do their marks. I guess all companies can have less-than-friendly operators, and if you get one, contact HQ and complain.