The Imperial War Museum (London) contains a surprising element in its First World War exhibit: a British anti-tank explosive. The description mentioned that it was designed to be used against British tanks that had been captured by the Germans and put back into service... This wouldn't have been as unusual as it sounds, given the high number of British tanks that foundered or were rendered helpless by mechanical failures, which were then repaired and refitted after capture. These captured tanks were organized into German units.

Here is link to an order-of-battle for the Schwere Kampfwagen-Abteilung, Deutsches Kaiserreich. ("Heavy Armored Vehicle Division of the Imperial German Army") It's in German, but you can get the gist of it, and see the photos at least. There's an interesting photo of a German repair workshop for captured British tanks. (Reparaturwerkstatt des Bay. Armee-Kraftwagen-Parks Nr. 20 in Charleroi).

https://de.wikipedia.org/wiki/Schwere_Kampfwagen-Abteilung_(Deutsches_Kaiserreich)

(I know that a wikipedia cite is usually unacceptable, but given the obscurity of the topic, I hope that the moderators might forgive this trespass in this instance?)

That page also mentions one English-language book:
Rainer Strasheim Beute Tanks. British Tanks in German Service. Vol. 1. Tankograd Publishing. Verlag Jochen Vollert. Erlangen 2011. But I have not read this book, so cannot vouch for it.

I do know that a German Freikorps operated two captured British Mark IV tanks in 1919, using them against the Communists in the Spartacist uprising. Here is a brief description (in English):

https://gottmituns.net/2012/12/22/british-steel-mk-iv-tank-in-berlin-germany-1919/