IPhone development - retainer

General information for the app: destination app
Kind of development: Customization of existing app
Description of requirements/features: I am looking for iPhone developer for regular work on a retainer basis.
The work includes:
- modifications of existing code (i.e modifying or adding new layouts to the app)
- integration of various SDK's in the code, and in some cases their customization (i.e. customizing offline map engine to display different map area)
- build packaging and App Store submission

The monthly retainer is 300 GBP.
1. Please let me know how many work hours can you deliver within the specified retainer?
2. What is your hourly rate for additional work outside the retainer quota (in case we have more work for specific month)

Based on proposal as specified above, I'll send a code example with few additional details.
Extra notes:
Thanks everyone for your time in submitting the proposals
To clarify: Next week I will send a code example with some follow up questions, but only to those developers whose proposals contain competitive pricing . Based on the answers received from this second round I will make my final choice.