What should you do if your neighbours’ dropped fruit is attracting wasps to your garden?

The first thing to do is to talk to your neighbours and see if a sensible resolution can be agreed to reduce and/or stop the wasps becoming pests. If they have no appetite to do anything then you can take action to remove branches that are traversing the boundary, in order to reduce the amounts of wasps on your side. Your neighbours do not technically need to do anything with any windfall on their side of the boundary unless their refusal is causing vermin to come into your garden, in which case it is possible this may be deemed a nuisance. However, this will vary depending on the fact and degree of the nuisance. It is always better to discuss matters with your neighbours or find any alternative solution that would assist in reducing the amount and frequency that the wasps come into your garden.

Are you within your legal rights to ask them to remove it? What can you do if they say no?

Answered above. If they have no appetite to do anything then you can take action to remove branches that are traversing the boundary, in order to reduce the amounts of wasps on your side.