Don't forget that "way-leave" agreements are seriously frowned upon in the fire safety world. Routes through neighbouring premises are fraught with problems - unfamiliar routes, routes secured shut, inadequate lighting, inadequate signage, obstructed routes, routes never walked during training sessions.
If at all possible it would be advantageous to try to establish safe routes that do not rely upon the neighbours, extended travel distances or not. Frankly, the extended travel distances are likely to be more acceptable than the routes through the neighbour's premises.
I do understand, though, that sometimes there is no alternative to the way-leave agreement. The best way to secure their availability to each tenant is to have the agreement included in the terms of the tenancy, the lease. By this means it is also possible to include terms that will secure the availability of the route at all material times. A solicitor may be able to draw it up but is unlikely to understand the terms that should be included in order to ensure that the route can always be safely used - I would suggest that a competent fire risk assessor can offer the best advice as to what precautionary terms should be included.
Even with the way-leave agreement in place the route is no good if staff are not periodically familiarised with the route. It is the place of the FRA to check that the correct training takes place.
Stu