Lease liabilities

We have a client who personally owns a commercial property that is let to his limited company. This commenced on 1 February 2004. The rent charged at market value (professionally valued) is £20 000 per annum. No VAT is involved and there is no formal lease.
If there was a formal lease a stamp duty land tax liability would arise at inception. In this case in the absence of a formal lease at what point does a stamp duty land tax liability arise and when and if relevant at what frequency does this trigger a requirement to file returns? As an alternative is a formal lease recommended?
This is also a common situation for owner-occupied properties and if there is a requirement to file returns is this the accountant's responsibility to bring this to the attention of his clients? And what is the position if the...