Yes you have the right to be upset.
Firstly do you have a gas safety certificate? Your LL should have given you one when you moved in.
Do you use an agent or are you renting direct from the LL?
Your gas bill is that expensive because of the leak.
The leak should be dealt with asap. As in today. Ring your LL and tell him the repairs are urgent. If he is abroad and there is no one left in his place, get the work done and when LL gets back get him to pay or reduce your rent by that amount.
The other things like insects - wait untill LL is back, write a list of things that need to be repaired/treated. If LL fails to action them, then to be honest I would be looking to move at the end of your lease. If you are on a long lease the these would be grounds to break the lease early.
The sheep in the field - you need to check exactly what is in your tenancy about the use of the land. Do you have 'exclusive' use of the field in writing, or were you just told by the LL, if its the second let it go - not worth the energy to fight.
Bats - are protected neither LL or you can do anything about them. Mice would probably come under your duties - housekeeping and maintenence. They are a 'feature' of country life. Ask LL to make the house as mouseproof as he can but mice are an ongoing battle that can IME can be controlled rather than eradicated in country houses.
Go and buy a carbon monoxide alarm for you own piece of mind.
If your LL wants to be an arse, don't forget he has broken the law by renting out a house without a gas certificate (and you wouldn't get one without checking for leaks, old appliances etc) and you can report him for this.
Also, try and get everything that you need to be done put together on one list so that you can give it to him on Tuesday, rather than constant dribs and drabs (it shouldn't matter but if he is as much of an arse as he sounds, then this may be the better way to deal with it).