The simple answer is no, sometimes in simple establishments this is easier as you are aware of what is specific to yourself. You can buy fire warden DVD training I use one fron the Fire Protection Association, this covers the role of the fire warden, trainers handbook, supporting materials including a quiz at the end.  Take a walk around your premises and look at how you deal with fires at present and make your training bespoke to yourselves. There are a lot of DVD's that show types and how to use extinguishers etc.

Article 21 of The Fire Safety Order 2005 is not prescriptive in who can train as long as adequate training is given, but it would be worth looking at it to ensure that you cover all the aspects that are stated under this article in your training.

I would do it myself, you have control over the content and how it is delivered.  This means that you can use your own policies and procedures and make sure your people know and understand how they should react and what they should do.  I would include a fairly large practical element to the course so that your fire marshalls can practice what they need to do without the stress of it being under either a fire situation or a drill where they have to deal with employees.