Sorry Jeremy, I cannot fathom your post. Are the readings all discharges?
Analogue meters can be quite inaccurate but a true 6.4 volt implies battery is reasonably charged somehow.

If dynamo not working ammeter should not significantly alter after engine starts.

As with other similar enquiries suggest you peruse the introductory articles on Club websites, and many old posts about not charging. And final understanding will most likely be better assisted by the Cornforth circuit diagram (typical) rather than from Austin wiring diagrams.

Assuming all as original the red terminal is the output D (early cars were -ve earth)

Location: Auckland, NZ

Watch out for digital meters and differing polarities (they don't like being upset by large currents going in the wrong direction). Across the battery will give you a voltage (engine off) reading but if the charge is going into the meter back to front then those little diodes inside your precious little meter are going to get swamped by excess current and questionable polarity!

The SB5 is the relay/contactor model number which was fitted to the CF3 cut out which had 4 terminals and 1 fuse and the CFR cut out which had 6 terminals and 2 fuses amongst others. Originally the RN was fitted with the C35A dynamo which had a box on it's top housing the field resistor, field fuse and the 2 dynamo terminals connected to a CF3 cut out. The dynamo terminal nearest the engine is the D terminal which should be connected to the SM5 switch panel D terminal. The other, furthest from the engine, is the F terminal which should be connected to the SM5 switch panel SH terminal. The A and D terminals on the SM5 switch panel are connected to the CF3 cut out A and D terminals respectively. For the cut out to work it must be earthed at it's E terminal. One of the reasons for dynamo poor performance is poor connections in the dynamo terminal box and SM5 switch panel. To save your knuckles remove the fan belt while testing the dynamo in place.
You say " I connect terminal A and D the meter shows 6 volt charge" would suggest the dynamo is doing it's stuff, but not charging.
You go on to say "When I connect the two terminals I get a 1 amp reading. The cut out does not appear to move and connect The cut out does not appear to move and connect" Where are you getting the 1 amp reading, most basic meters will not handle anything more than a few milliamps?

Think it's the brushes. Worn down and commutator filthy. The brushes I've seen are just carbon pieces, whereas mine have a short lead and a shoe. Also I could do with 3 pole shoe screws. Could someone point me in the right direction please. Ta Jez.

Brushes etc are available from our cherished suppliers. Click on the Austin Logo at the top of the page and then "cherished suppliers" and scroll down for info.