So today I find a card through my door from the Sspca wanting to inspect my animals..
If they do come knocking you have 2 choices:
1. Polite and firm 'No thank you.' And close the door / walk away. If they persist send them a written complaint for harassment.
2. Answer questions, let them see you're a responsible owner, and that there's no problem.
Personally I like the first option. They have no more right to enter your property, or handle or remove your livestock than any ordinary member of the public. If you let them in you're assuming they are going to be reasonable, and there are no guarantees that'll happen.
Without a police warrant they cannot insist on anything.
If they come back with the police, make sure they have a warrant. They almost certainly won't in your case.
If they do, check what the warrant says - it is unlikely to name the SSPCA officer(s), so you can continue to refuse them entry. The police will ask some questions, have a look around and leave.
And film it on your phone too - then we can all watch afterwards.