A pertinent subject for diesels but not an LR. Recently I had troubles starting my 20 year old Pajero. The batteries which were good twin Bosch silvers were 10 years old and past their 'sell by' date and failed to hold a full charge, so I bought new batteries. Then I still had issues, it took 5-10 seconds of cranking the engine before it fired up on cold days. The glow plugs were cheapo Ebay aquisitions and were not performing. I then bought some new NGK glow plugs, hell, what an improvement, the 'ole truck fires up now in a millisecond even in sub zero temperatures. The moral is not to buy cheapo glow plugs as they don't perform at all well or even last.

I have also found this in the past with spark plugs, I've always used the best NGK iridium spark plugs particularly in my Honda motorbike. They start it first time every time and last far longer than many others.