Are you with the Co-op? They are the only insurance provider whom we have heard about on here before who don't provide it and won't add it.
If you are with another provider, do read your policy wording carefully. It is usually in there somewhere. If you can't find it, then you may need to cancel the policy and take one out with someone else.
JohnLewis says:
you are covered for occupiers', personal and employers' liability up to £5 million for amounts which you or your family legally have to pay as damages in relation to: accidental bodily injury to any person
accidental loss of or damage to material property
accidental obstruction, trespass or nuisance resulting in interference with or loss of enjoyment of material property.
So if you are stuck for an insurer, contact JL as they will do it.
Certainly do call your insurer and tell them that you would like to cancel your policy as they don't provide employers liability cover for a domestic employee. See what they say - they may suddenly realise that they do actually cover that up to say £5 million, which is perfectly fine for you.
If your insurer won't provide it and you don't want to change insurance provider, then perhaps ask Hiscox or DirectLineBusiness and see what they say. You are an employer but you are not really a business, so I don't know if they would or would not provide you with a policy.