For the most part, sellers think buyerz got a bargain and buyers think they overpaid.
The buyers might be looking at the purchase and thinking they can not afford the necessary repairs at the price they are paying or they might see it as a part of the bargaining process.
How does you house compare to sold prices for similar properties in the area?
Old fuse box... I might consider replacing that... should be in the region of a few hundred pounds.
No one plans to rewire without planning on total redecoration. .. if they are planning that.. they can afford the price they are paying. You could get an independent electrical survey and see what that says... look for safety risks, urgent work and compare that to works required to bring the installations up to current standards... only safety and urgent works really need to be considered.
A quick google tells me that the average roof has a life of somewhere between 25 and 70 years.. in my experience.. they last much longer than that but need regular maintenance to replace missing / broken / moved tiles, reseat ridge tiles, repair / replace valleys, refit gutters, repairs to fascia / soffits, if the slates are laid on a membrane and this fails or the slats / beams are rotting.. then this would be a much larger concern. Most roofs will need maintenance and ultimate replacement at some point, but there are many many houses with roofs that are much older than 25 or even 70 years old.
I don't really see that houses with new roofs are worth significantly more than houses with old roofs... ie.. you wouldn't recover your investment in the short term if you put on a new roof and neither would they. .. so it's a bit of a gamble.
What could be determined from the viewing / visual inspection? The approximate age of the roof, the fact that it is in reasonable condition with no obvious leakage / damp. The age of the fuse box and the approximate age of the electrical fittings. The house is valued and offer accepted based on the age and condition of the house. If you can stand by your valuation, you can ask that your estate agent go back to them with that information and also offer to have someone look at the roof and electrical installations and see if there are any urgent repairs required.
If you think you can get another buyer... refuse the reduction. If you think they will walk away... how much is it worth to you to keep them as a buyer to meet your long term goal of moving house?