Sorry to hear about your problem Mick, re paying for the return I would just tell them them to neck them as if you pay it will be open season on your loft, sad but that is a huge possibility. Threats are a means of frightening people and if you live in fear of them they have won, 99.9% of the time they are never carried out because if I wanted to do something I would not alert you or incriminate myself. Defend you and yours as best as you can, if the scum bags are in that tiny minority that are willing to do it you won't stop them you can only deter them as best as you can.
I would do anything to help anyone but I have strong views and opinions and for that I have been threaten with my life and my property so I know how you feel, don't let them win by making you a prisoner in your own home.
Alert the cavalry but don't wait on them if you catch them deal with them as best as you can and deal with the consequences, they only act on evidence and take it further if they can secure a conviction, I have worked with the police on personal safety issues and you must play the scum bags at their own game"if you have a baseball bat then keep a base ball beside it" you and jack play all the time in the back garden, I know because I played with him last time I visited you. that is how the scum bags cover them selves and end up suing you for damages when you catch them
MY thoughts and prayers are with you and your family Mick and wish you all best
Duncan