My understanding is that you only have to tell the DVLA if you go onto insulin. Diabetes controlled by diet and/or tablets does not have to be notified. You should, however, tell your insurance company, as this might otherwise be an excuse they use to refuse a claim.
And since Gliclazide can cause hypos, you need to make sure your blood glucose level is safe before driving and also take steps to cover yourself against the accusation that you drove whilst hypoglycaemic.... so always take a BG reading before driving, and at regular intervals during any long driving session. The meter will date stamp and record the reading, and this can be used as evidence of your BG level in the event that you were involved in a road accident. The fact that you need to do blood tests for safety reasons may help persuade your doctor to prescribe testing strips... it did in my case.
Should you need to renew your driving licence or have it re-issued for any reason, its wise to declare your diabetes (tablet controlled)... then they have no comeback at a later date.