Andy,
Can you see the raw input for NOTAM PLOT?
As follows...
Ref: H2543/04
FIR: EGTT
Code: WELW
Traffic: VFR IFR
Purpose: PIB entry Operationally significant for flights
Scope: Nav Warning
Lower limit (FL): 020
Upper limit (FL): 240
Centre and radius (nm): 5157N00035E113
Parent ICAO: EGTT
Start date/time: 22/04/2004 10:30 UTC
End date/time: 13/05/2004 13:30 UTC
Activity period: 1030-1330
Lower height limit: 020
Upper height limit: 240
AUS 04-04-0321/1077/AS3
TLP MISSION. UP TO 32 FAST JET ACFT WILL CONDUCT HIGH ENERGY
MANOEUVRES WI 5322N 00130W 5338N 00030E 5328N 00230E 5237N 00253E
5213N 00235E 5213N 00022W ORIGIN. 5320N 00151E 5230N 00247E 5130N
00200E 5107N 00200E 5107N 00200E 5100N 00127E 5045N 00127E 5006N
00000E 5037N 00040W 5045N 00030W 5041N 00030E ORIGIN. ACFT WILL
REMAIN CLEAR OF REGULATED AIRSPACE BUT MAY BE UNABLE TO COMPLY WITH
RULES OF AIR. CONTACT 0032 716 81515.
I agree it isn't at all clear how one is meant to tell which subsets of the list of co-ordinates actually bound a complete area. It would be interesting to know what algorithm NotamPlot is applying. Its result does look more reasonable, but if I'd plotted it myself I'd have come up with exactly the same answer as you did.
BTW - must go on another outing once we've cracked these stealth NOTAMs!