@lucas @JaxVent I'm with @drandrewv2 on this in that I'd go with Prospect (and like her I'm a former Prospect rep - stopped when I moved to a non-unionised employer). Specifically because relevant to @lucas 's point, Prospect have members at National Grid (as well as being good for tech in general).
On the reviews, I'd say:
* The local branch does the heavy lifting. You will have a good experience if you have a good branch
* Prospect is less militant than some other unions in general. If a fight is called for, fine, but it's not the first tool in the box. Some won't like that.
* Prospect as I recall came out against the 999 operators' strikes a year or two back (Prospect represents managers at BT but not rank and file staff, I believe). That didn't go down well in some quarters, might be some bad reviews from that.