Letting house admin fees due to mortgage company.

I bought my house and due to me loosing my job,I have had to lease out my property, as I could not afford the mortgage,I have went through the proper channels using a letting agent, I have also asked for my mortgage companies permission, which the y have agreed to.They are now wanting £300.00 admin fees? which I feel is a bit steep, how do they come to that figure and why are they charging me this amount, what actually is the the admin fees for? and do I need to pay this? - Is this your question? Add additional information