Hi I think you are on the right track with Autoglym.

I have used super resin polish followed by Extra Gloss Protection on my Mercs Audis and BMW's over the last 10 years and maintain it with the shampoo you are using.

Wash the car. Clean it all over with white spirit when dry.then Super Resin polish followed up by Extra Gloss protection. A dry overcast day is best and go easy with polish you only need a little. I do mine Oct/ Nov and April/ May. Twice a year. If there is loads of salt about in winter just wash and reapply EGP in Feb.

The EGP is strange stuff like petrol. leave on as long as poss before buffing. I find a cotton cloth to buff off then a a good piece of silk really does the trick ( check out oxfam shop for a cheap silk item off the womens clothing rail but dont tell your mates!

Good luck

Grant)