Remove gas/oil heater - Do I need permission from freeholder?

No idea but you could always ask them

This is something that you should really check against your lease...However i know from my partner owning a lease hold as well as myself haveing a leasehold property- plus several other jobs i've worked on, that generally you can fit a new combi boiler etc remove a cylinder etc from within your own leasehold flat/property. i'm 99.9 certain that this is something you could carry out without notification to the freeholder. In fact often leaseholders can run their own Right To Manage company's so they have the say as to works that can be carried to the main property, such as communal decorating, re-pointing, roofing etc.