I recently bought a preowned Saracen Havoc 2000 (Team Saracen edition) and had to replace the original forks (Zokes branded double crown) as they were seized solid. Replaced them with an old but working set of RST Gila forks but now have a fair bit of flex where it moves a few mm's when rocked froward with full front brakes. Also makes a distinct knocking noise, as if something isnt fitting properly. Any help would be very grateful.
Sounds like your headset is loose. Have you properly rebuilt the stack order of the bearings, cups, shims and lower race?
Also make sure your star nut / top cap is pretty snug with the stem bolts loose, so you can fully seat the headset stack. Then tighten the stem bolts on the steerer tube.