John,
Here's the information and application form from the Philippine Embassy in UK.
There are some notes attached explaining how to apply by post if you cannot attend in person
A Single entry visa valid for 3 months from date of issue costs £22.00
Alternatively, you could purchase a visa extension at the airport upon arrival in Philippines. There is an office to do this in the immigration area but before you pass the immigration officer. Many have done this before and not reported any problems.