Heya mad1
It's worrying when one of our family is ill, and it's very easy to think the worst, but if you have a positive and supportive attitude things should work out okay.
The above explanantions of blood pressure are all excellent, and I couldn't really add to them in any way. The one thing I would say is not to go looking up treatments on the internet for a long period of time. There's a lot of information out there, and quite a lot of it is not true. Your best source of information is your local friendly GP who should be more than happy to answer any questions you might have.
When a patient comes to their doctor with symptoms of high blood pressure (hypertension), readings are usually made over a three month period or longer, so that it's not down to chance alone. Usually, depending on the age of the person, their diastolic blood pressure (the lower value)should ideally be less than 90mmHg. Their systolic value should be less than 160mmHg, but unless it is extremely high, treatment on the basis of it alone is rarely commenced. Above that, it's recommended that some form of treatment should begin. There are several causes of hypertension, some more understood than others. You said your mum has thyroid disease - this can contribute to high blood pressure. Sometimes people can get high blood pressure for no reason whatsoever (called "essential hypertension"). Thyroid disease can also make hairs very fine and brittle, so it's common for people to find that brushing their hair can damage it.
Okay now, so what can be done about high blood pressure? The most important thing for people to do is to take an active role in their control - to not rely upon medicines alone to deal with it. Some of the practical ways a person can actively help to reduce their BP include:
Weight loss through reduced fat and calorie intake (ie healthy eating - no Ulster fries!)
Regular dynamic physical exercise (eg brisk walking for 20 minutes per day)
Reduced use of salt in food preparation and elimination of excessively salty food (max 5g per day)
Increased intake of fruit and vegetables (7 portions per day)
Limit alcohol consumption (<3 units a day for men and <2 units a day for women)
Of course, as with all things, it's best for your mum to talk to her GP or Practice Nurse to see if all of these are okay (particularly if she's diabetic or on a special diet) before she sets off on any 10 mile hikes!
The medication that she's on at the minute sound like (i)Bendrofluazide, which is a diuretic drug ie it makes you pee more often. It's like plumbing, if you try to force water into pipes the pressure will rise, so by reducing the total fluid volume within the circulation, it is hoped that the pressure of that volume will decrease also. One problem with them is that your mum may be running to the toilet at all hours. The second drug, which I think you mean Atenolol, is a Beta-Blocker. These are highly effective drugs which work in a number of ways - they can slow you heart down a little so it's not pumping as hard; they can decrease your body's reaction to a lower blood pressure and so set its natural limits a little lower. Again your mum will have to be careful because they can make a person's pressure drop a little too low and they can feel light headed or dizzy if they stand up too quickly. She may also find herself getting a little more tired when she is on them, as well as her hands and feet feeling a little colder. Sometimes people also say that they get nightmares when using beta blockers.
The two together are a recommended treatment for high blood pressure.
Most of all, I would say to try and stop your mum from worrying too much - it can raise her blood pressure up even more. It would even be better for her to go away on a holiday - if she were to stay at home she might worry even more; at least when she is away she can take her mind off things which can only help the situation.
I hope this of a little help.