The second issue related to planning consent is the one you mention about distances and the bulk of a new building in relation to its surroundings. This is a planning judgement thing - no 'rules' as such. When the planning application goes in (you should get to know because you are affected), make an appointment to see the planning officer and raise your concerns and talk to them about the development. You will have to formally write an objection.
A second, more likely way you can influence this relates to the covenant over your land (the right of access). This is typically transferable to the new owner but the wording of it is crucially important. You are almost certainly going to need proper legal help with this but you can probably first the process for free by speaking to CAB to trying to get 30 minutes free solicitor time via them. Take a copy of the land title for the property next door with you that describes the wording of the covenant. You can download that here for £4.
http://www.landregist...ortal/Property_Search