Hi Joe
I would alway fit a new rubber mate makes the job of fitting the glass easier! they are not expensive either. There are many old wives tails on how to fit Mini screens most of which are rubbish
The best way to do one is NOT with string or washing up liquid 
I use class cleaner! fit the new rubber to the frame first (a little tape at the top for now to stop it hanging down) start at one lower corner and insert the class...the opposite corner will start a little hight up the frame so slowly wiggle it down into the corner (add class cleaner as a lubricant as needed) the next step is to get one of the top corners in!! the advise here is don't use anything metal to lever the glass in place! A plastic ice scraper works well...then slowly work your way across the top, I find it easier if some one can be inside the car pushing the screen rubber out. Take your time and all will be good. The filler strip or lock strip does require a special tool the insert but this can be made quite easy with an old wire coat hanger, make a diamond shape about 1cm across the widest part twist the ends together and add some tape or cut of about 14cm of a broom handle and drill a hole in the end for the wire ends to sit in and clue it in place. Make a simple job of fitting the filler. When it's all in a quick squirt with the class clean...wipe down and your done. Hope this helps
Oz
Oz Cooper
Classic Mini Product Manager UK
Helpful Links:
Code of ConductFAQsMoss Europe LtdDisclaimer: Working on automobiles is inherently dangerous. Moss Motors, Ltd. is not liable for injury or damage due to incorrect installation or use of their products. All products are sold with the understanding that the safe and proper installation and use of the products is the customer’s responsibility. Follow factory workshop manual procedures and instructions, but use current shop safety standards and common sense. Some tasks will require professional advice or services which Moss Motors cannot provide.