I know everyone is suggesting to ring about a million resuce shelters all over the place to come and get him out. But I really can't see how anyone from rescues would be able to do anything more to help - other then setting up the veggie traps themselves. (Besides the fact that the RSPCA already said they won't come unless it's injured :sad

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If anything, having heaps of people in front of the deck there trying to help bring him out would probably just scare Cookie even more, making him go further under the deck away from all the noise and people. I don't know, I could be wrong, I can't say I've ever had had a guinea pig stuck under a deck before

but I really think making Cookie feel as though it's safe to come out (and safe to eat all the veggies!), either with the pipe idea or just blocking off the hole straight after he comes out so he can't get back in, would have the best chance of success.
Loud noises and heaps of people talking definitely scare the hell out of my pigs, especially when they were new to their surroundings - which Cookie is. I reckon if you just hide (really quietly) on top of the deck or set up some sort of trap like the pipe idea, and wait for him to come out and get some more veggies again. There's no way you're going to be able to force him out without freaking him out even more and making him want to stay under the deck where (in Cookie's eyes) it's safe and protected.