Vauxhall astra 2ltr cdti dpf help please

Hi there I have a vauxhall astra 2 ltr ,the problem that I am having is I think the dpf filter, there seems to be a cycle firstly firstly the car will run great for a few weeks then when I turn the engine off the fan starts to come on and runs for a couple of minutes quite loud. Then a couple of weeks later the dpf light comes on and tells me to keep driving for regen to begin I do this & it clears it for a few weeks then it starts all over again I have sat on my drive and done it at over 3000rpm for 15-20 mins it clears it but no black smoke coming out ?? , I know I dont do the mileage for a diesel and I do lots of short journeys , has anyone heard of dongles that I can plug in th overide dpf ?? Really nice car great drive but so fed up with it thinking of getting rid of it, oh yes its a 13 plate auto any help or advice welcome, halfords offering dpf clean £85 am I just wasting my money help please