I would talk to ACAS about this one because I agree that your employer is acting unreasonably due to the length of time he is taking to investigate.
http://www.acas.org.u...x.aspx?articleid=1410
One wonders whether trade is light at present and he is seeking to save money by having you off the incentivised payroll? - otherwise I don't see how this action advantages him - because the nature of a sales role (where a large proportion of the income is commission-based) is that you generate income for the business, as well as generating income for yourself.
There is no maximum time it should take to invesatigate but the employers action have to be reasonable.
I also believe that your salary and benefits package does NOT include commission earnings (or average commission earnings) if you are 'off' - unless anything written down in an employment contract says something different (unlikely in such a business, I imagine). You could check with ACAS to make sure.
If your holiday pay is calculated on average commission earnings - that might be an indicator that you could claim that 'base level pay' included some commission-element?