if the WD40 doesn't work you should be able to mend it pretty easily.
you need to remove the stock first, make sure it's not cocked. there's a couple of screws up through the trigger guard that need to come out, and IIRC, one large screw that goes up through the bottom of the fore-end of the stock, that needs to come out too.
the metal parts should come away from the wooden parts now.
once you've got the rifle in two have a look at the trigger, there's two pins going through the body of the rifle just above the trigger that hold the trigger unit into the rifle, these need to come out so tap them through gently with a hammer and a nail with the end flattened down (or something similar). they should come out pretty easily, if they don't then stop and go to a gunsmith.
once the pins are out the trigger unit should slide out nice and easy.
the trigger is a sealed unit so you don't have to worry about anything flying off or coming loose.
once the trigger unit is out you should be able to slide the safety catch out of the back of the rifle, there should be a small spring behind the safety catch, try not to loose this.
it's possible on a second hand gun that the spring won't be there at all, some people remove them.
it's also possible that the spring has broken with age, a lot of HW35's are getting on a bit now.
if there's no spring, or the spring's broken, then i'm sure you can work out what to do next so i won't insult your intelligence by pointing out the blindingly obvious.
if the spring is still there, and intact, give everything a good clean with a bit of wire wool or similar, add a spot of oil and put it all back together. (re-assembly is so simple that i won't insult your intelligence there either)
if it still doesn't work then i'm afraid i'm stumped and it's probably a trip to the gunsmiths for you.
hope that helps a bit.
stuart