Sp43w6hp red gun convergence problem

Like a lot of postings on this site i have a problem with convergence, but just the red gun. Seems worse on the bottom right of the sceen, is fine in the middle/left (or directly in front of the red run) and a little off on the far left. The blue and green are fine. Seems to me that the red just cant focus correctly. Carefully taking the front off the TV I can see that there are four resisistors?? LDRs?? (not sure what they are) in the middle of each side of the screen. I guess these are used to focus the guns when I use the auto convergence option as each gun in turn fires lines across the screen which intersect these components. Funnily enough the autoconvergence always gets stuck and fails on the red gun cycle. If one of these failing is the problem does anyone know what they are called and how much they cost - it looks like I'm going to have to go to an engineer about this but i would like to know what the likely cost is before I get them out! Many thanks :)