Vokera 12-48 Mynute R.S.E ticking and blowing limit stats?

Recently our boiler has started to make a very fast paced ticking or clicking sound during its heat cycle, we have had to replace two high limit stats as they have both failed, the second one within a month of the first going. The water pressure was also dropping and we were having to re-fil it every morning, however there are no leaks around the house, and nothing coming our of the overflow outside. The radiators are all functioning properly and getting to appropreate temperatures. My father has been trained to fit boilers, but not to diagnose fault and can not decide on what could be causing these problems, his best guess is that water is not passing through the heat exchanger quick enough, which could be down to a faulty pump or heat exchanger, but we thought it best to ask someone who knows before buying replacement parts.

Thanks,
Tom