I'm back now. I drank enough Pils and Sekt to float a ship and it seemed we were always eating. Good food. I put on weight but found some records as well.
I went to a flea market and a secondhand shop in Mulheim, but nothing worthwhile there, just Rex Gildo, Heino and other things to avoid.
Thanks to Gregk45 and his contacts I knew about this record shop in Duisburg and one of my brothers-in-law offered to drive me there.

Behind the counter are two racks full of 45s running right across the shop. After picking out a few records there I was let into the kitchen next to the counter where there was a record shelf full of some rarer stuff. I picked out some more and then they let me into the cellar. Thousands of records down there, but not so well sorted out. I could have stayed there for weeks, but my brother-in-law was patiently waiting for me, so I only had a quick look. This shop is well worth a visit if you are in the region. Very friendly people, they gave me and my brother-in-law a cup of coffee, the records were moderate in price and they trusted me to be left alone in the cellar. I'd go there again.
This is what I got, not all German, not all garage:
The Lords: Have A Drink On Me/Late Last Saturday Night
The Lords: Que Sera/Boom Boom
The Lords: John Brown's Body/Cut My Hair
The Lords: Gloryland/Raindreams [all The Lords b-sides are better from a garage perspective]
The New Lords: Radio/TV
The Rattles: Sticks And Stones/Bye Bye Johnny
Ronny & The Daytonas: GTO/Hot Rod Baby
The Renegades: Cadillac/Bad Bad Baby
The Monkees: I'm A Believer/Steppin' Stone [just to get the German PS]
John O'Hara & His Playboys: Stampfkartoffeln Tatera/Louie Louie [what were these Scots doing in Germany?]
Casey Jones & The Governors: Jack The Ripper/So Long Baby
Ola & The Janglers: Land Of 1000 Dances/Thinkin' Of You
I also bought some records in Singapore on the way over and few more in Kuching, Borneo, on the way back, including The Beatles, The Archies and local favourites like Nancy Sit and Maurice Patton & The Melodians. In relation to this forum the best finds were Judy Lee and here Chinese version of The Monkees' 'Mary Mary' and the Bryan Neale & The Easybeats ep from Singapore. Hard to get that last one.