Grateful for any thoughts should anyone else have come across a similar scenario...  A converted barn used as a showroom and a workplace with approximately 10 employees. Only one toilet is available for both customers and employees which obviously doesn't meet legal requirements.  The owner is now aware of his responsibilities but bearing in mind the remoteness of the barn and lack of space in creating another loo, there is no easy answer.  On top of which the barn is listed.  

If you do go down the 'portaloo' route then might be worth considering getting one which is accessible for all (eg wheelchair access etc) to cover the equality act regs too. Could be used/availabel to anyone to use but at leats  it would mean you have an option for all. No idea how much they cost in comparison to a standard unit but if it's a showroom I guess it's open to the public so should cater for all eventualites if you can.

Welfare Regs; Reg 21 

It is acceptable to have unisex toilets but the requirement is given in the ACoP to the regs which has a table showing how many toilets are needed according to the number of employees.  That is the basis for saying that one isn't enough here.