I've found this thread on another part of the forum, looks like we're not the only ones to have been in this situation!
http://www.eyeonspain.com/Secure/ForumPostsDesc.aspx?thread=5492&dev=FB46&name=New?%20%20Introduce%20yourself%20here
In short, the approach that Grupo Trampolin are taking is COMPLETELY ILLEGAL. Developers cannot demand payment for utilities being supplied by themselves prior to the Certificate of Habitation being available.
In the case of other developments that encountered the same situation with their builders, the police and OMIC (la Oficina Municipal de Información al Consumidor - Spain's consumer watchdog) were involved, which led to the withdrawal of the builder's demands.
If nothing else, the OMIC can provide independent arbitration in situations like this, without the need to go to court i.e. much quicker than the traditional legal route.
I cannot find a website for the OMIC in Murcia, but I've managed to get their address and phone no.
C/ Saavedra Fajardo S/N
30001 Murcia, Spain
+34 968 358 600
What does everyone else think?