As JonCris says, see a solicitor about the management company.
In the mean time, I would go and see your tenants personally (giving them 24hrs notice of course

), sit down and have a cup of tea and explain the situation. Then make arrangements for heaters/free-standing radiators for them until you can get all this sorted out. The reason they can start to withold rent is that the place is not considered habitable without heat, particularly in winter. If they take you to court, they are entitled to claim back a percentage of the rent they've paid while this problem is ongoing (N4B, distress? There's a term for it, can't recall off the top of my head).
If you do provide alternative heating for them (you should), get them to sign something that lists what you've given them and the action you've taken them, i.e.:
Problems with management company repairing heating, during problem tenants supplied with three oil heaters, (date), any further problems, landlord contactable on 020..... Signed, Landlord and both tenants.
Always leave a paper trail! It helps if things go pearshaped later...
Hope that helps...