Why wasn't the Focus designed with an access under the rear seat..............

Try to do it when the level is low !

Undo the filler and vent pipe, disconnect and remove charcoal canister at rear. Disconnect filter (good idea to replace it while tank is dropped). Support tank, undo strap bolt. Lower tank, disconnect pipes and wiring. Undo access retainer and remove top cover. Turn fuel module to release from bottom of tank.

Might have changed, but Ford use to only supply a complete 'fuel module' though its easy to dismantle. Good after market suppliers (FSE in UK) can supply a bare replacement pump. Any where else, check against original pump as there are subtle differences, for example, in outlet pipe position.

Martin