hi, corgi registered gas engineer here.
missprim is exactly right, is impossible to give accurate advice over the phone and most gas engineers would be happy to come out and give you a quick free survey and quote and to check your flue is in a safe condition.
please note however that an inset gas fire is going to cost more to run than an outset gas fire and will not give out as much heat either. not a problem if your mum has central heating too but could be a problem if its her main source of heat.
there are some very nice modern compact outset fires available and they will be quicker and cheaper to fit.
if you do go for an inset then we recommend a valor one such as the valor dream and with a top operated control which is much better for older people. the valors arent cheap however, around the �350-�400 mark but they are quality esp compared to crap like focal points from b&q
if you need anymore info please dont hesitate to ask.
and were in the birmingham area if thats any use to you.