How early can I get an MoT done to preserve the date?

I had my car MoTd six weeks earlier on Feb 15th (the car was registered on the 29th of March) than it was due as it was being serviced. Should/could they have added a month onto the MoT taking it up to March 15th?

In short, no. You can get an MoT up to a month (minus a day) before it runs out and keep the same renewal date. Six weeks, however, is stretching it a bit too far. The MoT station is within their rights to change the date of the renewal.