Does it require a service? Also, how long should the main spring and piston washer last?
AIf you have noticed no decrease in power but only an increase in group size, then it is unlikely the rifle will need a major overhaul.
The fault regarding accuracy is most probably due to the use of unsuitable pellets.
Your rifle is a true .22 size with a bore diameter of 5.6mm. If you have purchased pellets recently, it is certain they are 5.5mm diameter.
These smaller pellets will be under-size in the barrel of your Mark 3 and will not produce the same accuracy or consistency as would the correct 5.6mm pellets.
If you have been using mineral-based lubricant on the spring and washer then this could also affect the accuracy.
As your rifle has a leather piston washer, only leather oil or any other non-mineral oil, such as Neatsfoot or any animal or vegetable-based oil, should be used.
Never use standard gun oil, as it contains silicon, which could cause wear on the breech plug.
Correct mainsprings, if required, leather washers and correct lubricant are still available.
With normal usage mainsprings and washers should last for at least 10,000 shots before needing replacement.