There are two legal issues here which are worth considering but more facts would determine the extent to which each applies.
The first is the TUPE issue; you should clarify with your former client that they intend to honour their TUPE obligations in respect of the cleaner since they are presumably bringing the work which she did for you 'in house'. This established, you then have a clear basis on which to make a claim for any loss which you may incur in losing your employee to a former client.
I'm inclined to agree with the view that 18 months would be viewed as excessive by a court in terms of a restrictive covenant but I don't think that this necessarily negates the principle as a whole and I believe that a claim based on the costs of recruitment, interview, training and development of your staff would be reasonable given that this large company have deprived you of a business asset.
It comes to what you want, really. Proceeding with a claim (which I would imagine to be within the boundaries of the 'fast track' procedure) might jeopardise the future employment of your erstwhile employee which would be a pity but you have a right to earn money as well and the actions of the company aren't particularly friendly.