WordPress website tweaks and fixes.

General information for the website: We have a website that is live and being actively promoted. Small errors, tweaks and fixes are needed.
Kind of development: Customization of existing website
Description of requirements/features: Map display breaks about every two weeks.
Categories are not linking correctly.
Addition of a event calendar plugin/function.
Addition of custom fields (Present functionality but does not seem to be working properly).
Speed optimisation.
Extra services needed: Customization, Security
Extra notes: The selected provider needs to reside in the UK and be reliable. Unfortunately reliability has be lacking with past providers although their work was good.

A long term ongoing relationship would be preferable as the website is set to grow rapidly as we invest and promote.

Please provide details of your general availability and response times to fix a website problem.