Sendmail fails on first instantiation

Sendmail fails on first instantiation

I have created a form embedded into a webpage and used the sendmail function to email me "on success" when the submit button is clicked. The success of the sendmail function seems to be time dependent and shows the following behaviour:
1. Function fails if no form has been submitted for a time greater than 107mins (I am still testing to find if a shorter time causes failure)
2. Function works ok if a previous form has been submitted less than 62mins beforehand (I am still testing to find if a longer time is ok)

The form can be found at www.edinburghrc.co.uk/about/equipment/equipment-booking-form

Any help to resolve this would be most welcome
Regards
Graham