Dear [Name],
PROJECT NAME
We are obliged to comply with the RIBA Code of Conduct which prevents us from undertaking work without a written agreement as to the service offered and the fees to be charged. This letter provides a basis to comply with these requirements to cover the period until a more formal and explicit contract, as referred to below, can be executed.  
We have agreed to undertake [note scope of speculative element of the work] on a speculative basis, but should work proceed beyond this stage fees will be payable as set out below.
[Set out proposal for appointment should the project proceed further.]
If these terms are acceptable, please sign the enclosed copy of this letter and return it to us. We shall then be in a position to start work. We are looking forward to working with you on this project.  
Yours sincerely   
[Your name]
I/We confirm that [Practice name] is to proceed with the preliminary services as set out above. 
[Signed] 
[Client's name] [Date]