How to replace Sangamo Choice PR2 with Sundial RF2 Pack 2?

Hi all,
Would very much appreciate some help here.
We've moved to a new property that has a Grant Euroflame 70/90 oil-fired boiler and a Sangamo Choice PR2 two-channel programmer with cylinder stat but no room stat. The Sangamo has a standard backplate with a black wire for HW=ON (t3) and a grey wire for CH=ON (t4) and DIP switch set to gravity.
We would like to add a wireless room stat and Honeywell have suggested their Sundial RF2 Pack 2 would be a good replacement for the Sangamo.
Do I need to set the Sundial for gravity and if so, how?
Can I leave the backplate wires as they are and just swap in the Sundial?
For info, the previous owner said he used a Hive and just put the Sangamo back without needing to change any wiring.
Many thanks in anticipation of your help!

Quite possibly another dip switch, but have a look here http://www.free-instruction-manuals.com/pdf/pa_458128.pdf

Thanks Chris, but those docs don't suggest that there are any DIP switches on the Sundial

No it doesn’t seem to, however it does have a section stating remove links, which is only applicable to C-plan which I think yours might be.

Thanks again Chris.

Mea Culpa - the issue is more complicated than I thought -
Although the Sangamo programmer has its dip switches set to 'gravity', the system is actually fully pumped.
But there are two pumps (one CH, one DHW) and no zone valves - I'm told this was not unusual for oil-fired boilers on new builds in the 80s as it was deemed more reliable and easier to repair than replacing failed motorised valves.
So can I just replace the Sangamo with the Honeywell Sundial as though the system is a sort-of S-Plan (which I'm guessing is how the previous owner used a Hive and then reverted back to the Sangamo when he moved out)? Or will I need to have a relay fitted to keep the CH and DHW operating independently?