Excuse my ignorance, but does this basically mean that the wires need to be higher? If so does that mean taller masts/portals or can the series 1 stuff already installed accommodate that? (looks like it can from images on F&F website)
It's complicated.
The distance really only applies on platforms - passengers will generally always be more than 4m away from the nearest energised part of the OLE system elsewhere - such as level crossings, footpaths etc.
The distance also isn't actually 2.75m or 3.5m from the contact wire, but 2.75m/3.5m from the nearest energised part of the OLE collection system, this typically means the part of the system energised at 25kV closest to a passenger are the horns at the extreme ends of each pantograph.
The horns, btw, exist to prevent the pantograph riding above the contact wire, and to try and guide the pantograph back under the contact wire if there should be a significant sway/deviation event. That's just to head off any - just get rid of the horns chatter.
The calculation of 2.75m or 3.5m is also a little complicated, as it measures from the extreme edge of a platform to a fictional point adjacent to the contact wire where the nearest part of an energised but stationary pantograph can be found. The pantograph is allowed within the 3.5m envelope when it's moving, provided there's a yellow line provided for passengers to be required to stand behind for their own safety.
It is, as I understand it, a bit of a concession to the fact people now have larger umbrellas, essentially. They're both larger and more likely to be blown closer to the 'energised envelope'.
To create 3.5m space, it's only an issue at those stations with particularly low over bridges where the contact wire height has been dropped to avoid bridge rebuilding, tunnel reboring, that sort of thing and it's an issue that can typically be resolved by dropping platform and track height.
There are probably a few locations where it can also be resolved by reducing the height between catenary and contact wire, although in most cases, the contact wire should be as high as possible over the track and the catenary wire should be as close to the contact wire as is necessary to give as even a contact wire height along the line.
The 270mm to 370mm change is more of a problem, as it could result, feasibly, in areas where a 270mm clearance between catenary and structure gives 3.5m clearance between 'energised envelope' and platform edge, but dropping the catenary by 100mm reduces the 'energised envelope' to 3.4m, so risk assessment to keep 270mm clearance would be needed.