Hi SIMON !
Yes the PAL TV transmission system used in Europe displays 625 lines at a 50hz refresh rate, whereas the NTSC TV transmission system used in the USA/JAPAN and some parts of South America displays 525 lines with a 60 Hz refresh rate.
This used to be a BIG problem for displaying and recording NTSC originated material in Europe , since not only is the COLOUR system different (PAL/NTSC). The number of lines (525/625) and the refresh rates are ALSO different.
However since the advent of INTEGRATED CIRCUITS IC'S , its easy to make the required Timebase and Colour decoder chips include facilities for operating on both systems. There WAS an overlap period however when the chips did not support the FULL features of ALL the systems.
A common configuration for quite some while was for the TIMEBASE (Line/Refresh)chips to support 525 / 60 Hz displays, but for the COLOUR DECODER chips not to be able to decode the NTSC Colour.
This results in a FULLY LOCKED but BLACK AND WHITE display.
This sounds like the situation with you're cousins older television.
However , as you'll have noticed from what I've stated above . If the television is capable of giving a perfect BLACK AND WHITE picture when displaying NTSC sourced material .It is therefore capable of displaying sources which have a 60 Hz refresh rate.
Therefore judging by what the person at Maplins has said, the NTSC convertor should work fine with you're cousins TV
Hope this is of some assistance
Best of luck !
Scrumpy Jack !