Hello please could someone advise the cheapest fare from Penryn (Cornwall) to Whitehaven (Cumbria) with split ticketing? the best fare shown on NRE is £170.
Not really, no. We can never guarantee to find the cheapest combination, and it depends on how much you value time vs money. It depends on various factors such as what is the earliest you are prepared/able to depart, and what is the latest you can arrive by? It can also make a difference if you want a single or, if you are returning, is it a day return (if so, CDRs can be used) or a period return. If it's just a case of looking at splitting, then my advice is to look at the timetable - or a map - and then look at the fares for intermediate points.
There will be many intermediate points and it is impossible to check them all, so you just have to prioritise with certain key stations and check those. For example, Plymouth, Exeter, Taunton, Bristol, Cheltenham, and so on...
But the cheapest splits may well be via more circuitous routes such as Worcester.