Does anyone have a diagram of how a system with 2 pumps and no valves is
wired together to give interlock?

Very simple. Wire the programmer output through the appropriate room or
cylinder thermostat to the pump. The same connections should also be made to
a relay. The Normally Open outputs of the relay should be connected together
in parallel and be used to run the boiler interlock.

Secondly how does pump overrun work if 2 pumps are used (i assume you
have to dump into the CH cylinder)

This gets a little more complicated. You basically need the pump overrun
output of the boiler to run the hot water pump. However, it is essential
that this doesn't end up as a call for heat back at the interlock, or it
will never switch off. You also don't want the HWC pump to run when the CH
pump is running, or the HWC will overheat.

Let's draw a logic diagram:

Inputs:
HWC = hot water call for heat (from thermostat/programmer)
CHC = central heating call for heat (from thermostat/programmer)
POR = pump overrun from boiler

Output:
HWP = Hot water pump
CHP = CH pump
BIL = boiler interlock
(T) = should be impossible or transient condition only


HWC CHC POR | HWP CHP BIL
0 0 0 | 0 0 0
0 0 1 | 1 0 0
0 1 0 | 0 1 1 (T)
0 1 1 | 0 1 1
1 0 0 | 1 0 1 (T)
1 0 1 | 1 0 1
1 1 0 | 1 1 1 (T)
1 1 1 | 1 1 1

(Alternative for DHW priority)
1 1 0 | 1 0 1 (T)
1 1 1 | 1 0 1

This gives

HWP = HWC OR ( POR AND NOT CHC)
CHP = CHC
BIL = HWC OR CHC

So, connect CHC directly to the CHP pump.
BIL is run by paralleled relays from HWC and CHC.
HWP is harder, but can be done with a few relays.

To be honest, I'd be inclined the run the whole control system in low
voltage logic (i.e. 5V 74LS/74HC) and end up in a nice relay for the boiler.
But then, it would be simple for someone like me to throw together something
on a bit of stripboard with a 7805, a wallwart and a handful of SSI logic
chips.

Christian.