If in doubt, go and speak to a CFI at any of the aerodromes under Class D airspace. Or even better, why not ring the Doncaster approach/radar assistant and ask for information on how your flight will be affected by controlled airspace?
Sandtoft have done a newsletter with excellent informaton and suggestions on operating in to and out of the aerodrome without infringing Class D airspace including photos on the geographic features for circuit flying and joining/leaving the ATZ.