
The landlord or agents definitely do not have the right to enter your house without your permission, in fact, you don't actually have to give permission for viewings, the only things I beleive that HAS to be allowed at some point during your tenancy is entry for an engineer to do the relevant checks for a gas safety certficate once per year.
BTW, they can't withold your deposit becuase you won't let them in to do viewings. Your deposit should be held in one of the 3 protection schemes (which the LL/agent should have given you written details of), and they can only make deductions for damage that you have made during your stay, not for any personal 'gripe' They can also only make deductions if they have a fully signed check in inventory from the start of the tenancy.
You can change the lock barrels (quite an easy job with a screwdriver), provided you then change them back to the original ones at the end of the tenancy. Regardless of what your tenancy agreement says, it cannot override law or any acts that have been passed. This includes the right to quiet enjoyment of your home during the tenancy period. Besides, if you did change the locks, how would the landlord find out? When they tried to let themselves in at a time that you weren't home? Which they aren't allowed to do anyway.
The Moneysavingexpert website has a great section on house enting, with a lot of knowledgeable people, and there is also a National Landlords Association webiste, I'm not sure if the forum is memebers only, but they may be able to help as well.
It's a subject I've done a lot of digging into over the past year or so as my father has a couple of houses he rents out, so I did all of the background research for him to make sure that he was fully compliant with all of the various acts/laws. It is a bit of a minefield, and there are a lot of agents & landlords out there who either are ignorant of their responsibilities, or just don't care about them :-(
HTH - if anyone wants any more info etc on renting, I'm more thanhappy to help if I can :-)
Claire