Are the upstairs tenants willing to help you out with this? If so, surely they must have some method of contacting their landlord, and if not, I'm pretty sure it would be legal for them to have the work carried out, get receipts and withold that amount from their rent. Either the landlord would let it go without comment or would contact them about it, in which case they could present the receipts and let them know that you have been considering legal action.