Problems problems - It is now illegal in the UK to import or sell 100W lamps! The law just came in a couple of weeks ago... Probably via Brussels...
It really is the landlord's responsibility to do maintenance - not changing light bulbs of course but replacing light fittings.
A new light fitting with two 60W holders or similar might be better but it won't be long before 60W lamps become illegal too.
If I were in your situation I would ask the landlord if it's OK to change the light fitting - if she said Yes then I would go to a specialist lighting shop and seek their advice - they usually have a better range and more helpful and knowledgeable staff than the DIY supermarket stores.
A potential issue, depending on the style of the landlord and your relationship, is liability - if you go ahead and do the work and there's a subsequent problem she may hold you liabale for any repairs - tread carefully.
Good luck...
Peccavi
September 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lighting.