Hi Snibble,
Can't help with the sea Vixen , but for the Phantom, here goes:
Based on 1/72 plans in Warpaint No 31, the length of the 2 fully extended Oleos on the FG.1 undercarriage is about 125cm (Full size) from the base of the main nose U/C leg to the centre of the nosewheel. Obviously, as there are 2 extending Oleo sections, there are 2 sets of torque links to consider (On the front for the upper Oleo section and on the rear for the lower).
Converting into 1/48, the total length of the extended Oleos converts to about 26mm (From the base of the nose leg).
Other points to consder when modelling an FG.1 are:
The catapult hook aft of the underside of the Starboard engine intake,
3 lights on the nosewheel door in a vertical strip green at the top, amber, then red at the bottom.
No RHWR fairing on the vertical fin tip.
Finally and probably most importantly, the FG.1 featured a slotted stabilator, which may or may not be included in the Hasegawa kit!
I'd recommend you get a copy of Warpaint Series no 31 - McDonnell Douglas F-4K and F-4M Phantom by Steve Hazell, as, for me this is the definitive reference on UK Phantoms.
Hope this is of use,
Cheers,
Rob M.