Practica
I think the article quoted is referring to the system as opposed to the missile and in this respect the article is correct. The system (or GWS30 as it was known) consisted of a target acquisition radar Type 965 (WW 2 vintage and full of valves), 2 off Type 909 illumination radars (solid state I think?), an FM1600 computer (solid state using an F1000 integrated circuit CPU), the missile itself (solid state) and the associated launcher/magazine. The Type 965 radar was imposed as a stop gap/cost cutting measure and was gradually replaced by the more modern Type 1024 which is largely solid state I understand. But according to college of mine and a fromer Sea Dart development Engineer, the missile itself was definitely solid state electronics and totally maintenance free whilst at sea. Consider that the Bloodhound Mk2 missile, which pre-dated the Sea Dart design by about 10 years, also had a solid state seeker head, but the earlier Bloodhound Mk 1 used only valves. Apparently the Bloodhound Mk1 needed to be switched on for several minutes prior to launch so as to warm up the electronics! (you can see these seeker heads sectioned at the Bristol Aero collection – Kemble)

Interestingly the Missile itself is stored in the magazine dry, it was only filled with Kerosene in the second or so prior to blast off.

Sea Dart was once launched from a box by way of a demonstration, and vertical launching using TVC was explored in study form. I think this would have been problematic because in the boost phase you have to get the ramjet lit and this was tricky enough with the missile going in a straight line. As a missile, Sea Dart Mk 2 was only studied and never flown.