Apparently the BA guys were troubleshooting a hydraulic leak on one of the main gears. This required lifting the landing gear lever to the up position. This wouldn't normally be a problem if the nose gear downlock pin is correctly installed, but the pin rests on the nose gear doors and when the doors opened as the first part of the gear sequence, the pin fell out as it wasn't correctly latched into position, consequently the nose gear retracted.
The aircraft also suffered damage to an entry door which came down onto a set of steps, which also set off the escape slide.
Unfortunately, the aircraft also settled onto the fixed electrical ground power unit, which got buried into the fuselage.
So I think that this might be a tad expensive to repair!