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Firstly, the APN (Access Point Name) is only if you have a SIM card from the mobile provider that you using in India. It is part of setting up the mobile data configuration.
However as you are roaming there are two possibilites.
1) Since you only say that "I've tried all the obvious things" without elaborating I assume that you mean you have made sure that mobile data roaming is activated in your phone
2) It if has been activated in the phone then you will need to contact T Mobile back in the UK to activate it there.