If your interested I have a mate who is selling a rat catcher air rifle for around the £100 mark might be able to bargain him down some and I'll toss in a few C02 bulbs I have in my possession. It's been owned and used by my mate as his go to rat rifle for about 8 years so a few cosmetic dings but works like a charm.
Edit:
The MK4 is a nice trap, I have lost count how many I have had snap back on me after filing down the plate trigger a little to far, but for the love of god don't bait them it's treated as a crime and it also can cause problems with the kill as they are designed to crush the rib cage I have seen traps set incorrectly and seen Rat's and Rabbits (in a MK6) escape (before your horrified I finished them off with a .22 or a shotgun before they ran off wounded, nor did I watch them suffer I have seen the escapes when checking taps very few caught in the trap).
But not exactly useful in a garden environment, cat's tend to poke dead rats and the traps can close more than needed and trap and harm a cat. Poison isn't the answer for humane reasons, so shooting is the best answer in this case. In the past I would say though if you can bait them in if shooting, put out a bit of white card with a digestive soaked in the water from a tuna can or a stock cube diluted in water, you can pick and chose your targets.