With the garage, I guess due to time as well, demolishing & re-building is not an option before sale is completed.
I don't think I would be un-reasonable to drop our offer a tad, based on costs of removal & rebuild?
It's worth a try, especially as your surveyor was not allowed access to it. The question you need to ask, though, is whether it's worth walking away from the house over if you don't get anything off.
Worst case - you need to pay someone to get rid of the asbestos. Might cost you a grand on the cost of a new garage. It's probably relatively small money compared to whatever you've already spent doing searches, etc. let alone what the new garage will cost .. Get some pikeys in and turn a blind eye and it'll be less than that. ;-)
Same with the electrics. Most likely it's been updated several times since the '30's. If the current installation is 70's onwards, there's probably not a great deal to worry about from a practical perspective. I'd change the consumer unit if it's not a modern split RCD protected job but otherwise chances are you're not looking at an enourmous liability. If you can get an inspection paid for, and the delay is not going to cause the sale to go through, at least you know exactly what you're up for.
The other option might be getting indemnities against some of the potential problems - i.e. the seller buys an insurance policy that indemnifies you if the place gets eaten by woodworm - thus allowing you to continue with the purchase with some confidence.