You say you do not dispute the amount you owe them, so if it is owed and was not paid to scotish power then they would of legally sent it off to scottcall for collection. Based on this there is no need to ask for a signed copy of your agreement which they do not need to send anyway as they are not covered by the consumer credit act CCA.
Contact scotcall and come to a payment arrangement with them but only what you can afford, if they are pushing for more than you can afford let us know but better to post in the
debt forum, ppl there can advise you on arranging affordable payments with DCA'S and ensure scottcall are within the line of the law on collection. Good luck with it.