If all you worried about is the DD, then fixed doesnt stop it going up, during my time on Octopus they kept increasing my DD twice a year even on fixed deals, I have nearly £800 credit with them. These companies seem to have a business plan of building up cashflow by using customer's credit balances. A few days ago I gave them a reading which should give me a bill of around £600, so I could get as many units paid as I can on my last year plan which ends on Octo 5th. After Oct 5th I am on the SVR plan protected by ofgem cap.
You can change the DD amount via their website. They put mine up and I put it back down - what's the issue??
Edit, just noticed you aren't on SVR yet..