Take a 26mm set spanner
Check that the spanner has a clean, smooth inside face before placing it anywhere near the toggle valve
If the spanner is not smooth on the inside face you will mark the chrome
if you mark the chrome on the body of the toggle valve it is not an inexpensive fix – you either live with it or replace the whole toggle valve
for reasons not entirely clear to me the end caps on the valves have been done up extremely tigthly – i can not see any reason why they need to be so tight
this means you will need another 26mm set spanner saddled over the top of the toggle valve at the ‘T’ juntion to prevent the whole toggle valve turning as you undo the extremely tight end cap – dont let it turn or you are likely to create a leak on the inside of the casing where the copper pipe joins the back of the toggle valve