Our auto irrigation system wouldn't turn off. We found the Davey XJ70 pressure pump was leaking so replaced the leaking pipe. Now the watering g system won't start. What do we do?

Having a leaking boiler is understandably an inconvenience none of us need or want. The cause of a leak is usually due to 2 factors - the age of the boiler or poor work by the engineer. However, these things happen and it's important to take the right steps to diagnosing the problem sooner rather than later. Prolonging the issue can ultimately lead to further damaging the internal components of your boiler, making for a more expensive repair.
Pressure Valve

A boilers pressure should be sat at one bar on the pressure gauge. If the pressure exceeds this amount, the pressure relief valve should leak water to prevent your boiler from bursting. If for some reason it don't, it's likely you will encounter problems with other components on your boiler.
Poor Fitting Installation
Common with newly installed boilers, it's an easy mistake for engineers to make to not seal pipework correctly which ultimately causes the fittings to leak water.
Boiler Pump
When you incur a leak with your boiler pump, you have two options to resolve the issue. Replace the pump entirely or, reseal the pump. However, depending on how much an engineer charges to replace the seal, it may or may not be worth replacing the pump with a brand new one.
Expansion Vessel
Most plumbing systems will need an expansion vessel as they will need to contain the water as it expands when it heats up. However, in time the pressure will takes it's toll on the vessel and it's been known to split the vessel, leak from the hose and/or leak from the fitting where the braided hose joins onto the tank. If you encounter all three problems it is best to replace the expansion vessel and the braided hose. Furthermore, if you have a leak on the fitting then it may just be a case of a poor fitting by the previous engineer. Therefore, it wouldn't hurt to get an engineer to PTFE the joint and adequately tighten it back together correctly to ensure the system stays leak free.
Corrosion
Pipe corrosion is a common problem that can be cheap or some-what costly to fix in some circumstances. A cheap fix would include corrosion on a part that is small and easy to access and replace. An expensive situation would incur a wide-spread corrosion in multiple areas which would likely result in replacing your boiler.