New washer on MST ballcock - advice needed

Having replaced my cistern ballcock washer plenty of times, I thought doing the one in the MST would be the same - but I have been warned that it is not as easy! So what do I need? The house was built late 70's early eighties - is it likely to be a metal washer? I bought some 1/2" rubber ones thinking they would work...Any tips on doing this job would be much appreciated. Graham
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.