Severin Carrell is just being wilfully ignorant, and continues to peddle this myth in the Guardian whenever he can.

https://weegingerdug.wordpress.com/2017/03/11/la-gota-que-co...

"The argument upon which Spain bases it opposition to Catalan and Basque self-determination is a clause in the Spanish constitution which forbids them to have independence referendums. There is no such clause in the British constitution forbidding Scotland from holding an independence referendum. When Scottish independence come about, it will be legal, it will be constitutional, and it will be negotiated with and recognised by Westminster.

Spain doesn’t recognise the independence of Kosovo from Serbia because Kosovo declared independence unilaterally, an independence which isn’t recognised by Serbia and which Serbia claims is contrary to the Serbian constitution. But Spain does recognise the independence of Montenegro from its union with Serbia because Montenegrin independence was permitted by the constitution and is recognised by Serbia. Unionists only ever cite Kosovo, never Montenegro.

2013 puts that in the context of the 2014 Indyref, where the Spanish were against the possibility of independent Scotland being considered as a successor state of the UK + thus automatically gaining EU membership.

Nowadays the context is on independent Scotland applying to join as a non-EU member, which no one has made any objections to.