Might be best to approach your FTO directly, you are on the right lines but this should help. This JAA exam question was asked years ago and does cause some problems - you are not alone.
Scale is Earth Distance(ED) / Chart Length(CL) (as a ratio) saves all this 1 over rubbish. You must work in the same units Km/cm NOT Nm & cm.
Next thing is to spot it's a Mercator question so the 133cm CL applies at ALL latitudes so you can dismiss the 53N bit and go straight for ED at 30s. This brings in "departure". Dept (NM) = Ch long (mins) x cos lat.
So 360 * 60 * cos 30 = 18706 nm, however CL is in cm so convert 18706 into km by * 1.852 = 34643.512 km (worth remembering 100,000 cm in km) so * by 100,000 = 3464351200 then divide by the CL of 133 gives answer of 26,047,753.
The original exam answer was 1:25,000,000 which is close enough.
NB. Several questions on scale where you have to find ED by departure before you can do anything else.