Yes, you can. The Hive [you need the Dual Channel Version] would directly replace the Horstmann. The existing wires are simply moved from the terminals of the Horstmann to the Hive terminals that have exactly the same function.....
.....So based on the above diagrams, the wires in:
N = N
L = L
HW off (3) = Hot Water off (1) (This may, or may not have a wire in depending upon your plumbing arrangement)
HW on (1) = Hot Water on (3)
CH on (4) = Heating on (4)
The links at the Horstmann shown as dotted lines in the above diagram are discarded, the Hive dual channel version has these already made internally.
The wireless thermostat will go, but it can't just be disconnected, simply removing it would leave the heating wiring 'open circuit' and so it wouldn't work. The wiring to the Salus receiver would need a small modification to complete the circuit. When you get to this, if you post a photo of the wires connected to the Salus receiver someone will be able to assist.
Whilst this applies to Hive which you mentioned, the same principle applies for other manufacturers.
and a salus wireless thermostat controlling my tower central heating pump.
Are you sure about this bit? The room thermostat usually operates a motorised valve, unless you have a system that is 40 years old +