Samantha - If you had posted the make and model of your boiler, it may have been that someone would have had more idea of what to look for.
You would do well to look for the installation guide and user manual for your boiler. Do that There is often a fault finding guide and diagrams of the parts in the guides/manuals. Google search is often very productive.
You may well find plumbers forums that have your make/model mentioned. These are often a gold mine of practical experience based information.
Anon, May 2016