You could have a go yourself. The spring (if standard) isn't very strong- obviously make sure the thing's uncocked before starting. You need to remove the plastic cap at the rear of the cylinder, this is just a push fit. On removing this you will see the large diameter securing pin and what looks like a washer- actually the rear of the spring guide- you have to compress this to remove the pin. Don't just bash the pin, it has cutaways underneath which fit the rear of the spring guide and if you hit it hard enough one or the other component will be destroyed. I removed this by securing the barrel with several cable ties, resting the barrel on some old carpet and bearing down on the thing with a large flat bladed screwdriver- this method does carry a risk of damage to oneself and the rifle but take it steady and you'll be ok. When the pin is out you can remove the spring. Take the cable ties off the barrel which can now be broken to allow removal of the cocking shoe from it's slot. This just leaves the piston which will come out after some persuasion- don't damage its' front surface or the front circumference where there should be a sealing o ring fitted (probably square by now) I can't remember whether you have to depress the trigger to get the piston back past the sear. At this point you will have a small pile of components which can be inspected and cleaned and/ or replaced. I would say that you will probably need a new spring, new O ring piston seal, and new breech seal (the one fitted to the breech end of the barrel which will be useless - they turn to cheese!) Try and get an original spring, the square ones promise increased power but make the rifle feel rough to cock and shoot. You might try lightly polishing the interior of the cylinder, for this you need to fix a rag to a piece of dowel and use something like 'Brasso', just make sure you remove all the abrasive when you've done. I wouldn't do anything to the trigger assembly apart from giving it a squirt with something like car brake cleaner to get all the old oil and muck off then relubing it with light oil. To reassemble just reverse the dismantling proceedure, making sure no oil is left in the cylinder and no oil goes on the face of the piston- you might use a drop of silicone oil on your fingertip to smear around the new O ring to ease it back into place- this is the only place silicone oil has inside the rifle and should never be used on metal bearing surfaces. Once the piston is in place (don't forget the cocking shoe) you can put in your nicely greased new spring (get the correct grease from your gun shop) and spring guide. That's pretty much it, expect to work harder getting the new spring back in, especially if you use my bodge method - If you do it this way and have trouble holding the spring long enough to get the pin in don't try to put the pin in straightaway, use a smaller diameter screwdriver to shove through the holes and hold the spring in place temporarily- you can then get the pin in place in stages, withdrawing the smaller screwdriver as you get the pin across. If you are successful you will have the satisfaction of having done a good job and saved some money, if you are unsuccessful you will probably have a stab wound from the large screwdriver, maybe a black eye from a rogue spring, and a local gunshop owner who will try (but probably fail) to hide his amusement as he presents you with either a bill for fixing what you broke or the news that your rifle is now only good as the basis for a piece of modern sculpture. 
Good Luck Jon.
'Dieu n'est pas pour les gros battaillons, mais pour ceux tirent le mieux.'
BSA Meteor Super
Problem is it's as old as time and hasn't been used for years 
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Before I embarrass myself even further, what does the long-stroke thingy mean?? 
