Low temp/high rev problem cured since oil change and service reset??

Hi there have a 1.6tdi CAYC octavia and it had a couple of problems.

1. it took ages to reach 90C and tended to drop quickly when stopped for a short time and then take ages to get up to temp again. Rad fan would come on but only once engine was switched off even after a short journey with temp showing only 60C 

2. It tended to rev high for a good while untill temp hit 90C which it would only reach on a longish run. Revs were sitting at 1000-1200rpm until 90c then drop to about 950rpm

I changed oil plus filter and had a 200mile trip and now it warms up quickly and the high revs have disapeared. I only changed oil and filter and reset the service warning. Is this pure coincidence or is there something I am missing?? Not complaining but its puzzling me. I also changed a drop link,bearing and a couple of ball joints which shouldn't affect anything

Anyone come accross this or should I just be grateful its cured itself

On a sepearate issue who designed an engine that you have to remove electrics and stainless pipe so the oil filter will come out?? Its bad enough that they dont have a flap in the undertray for draining oil. Thats progress for you??

Alasdair