Before you buy insulation batts get up in the roof and measure the distance between the ceiling joists, batts come in 430 or 580 widths if I remember correctly.
I did my house a couple of years ago, It was a fairly cheap exercise but it was damn hard work.
Putting them up through the man hole was a non event, I found it much easier to pull a sheet of tin off the roof, as a bonus the sheet of tin off allowed some much needed airflow into the roof space.
I put an 25mm thick MDF lay board about a meter long buy 600mm wide in the roof to span the joints so I could kneel on it, and lay on it to get into the tighter spots, made it much easier to avoid putting my foot or knee through the ceiling.
I used Bradford batts and didn't find they made me that itchy, I didn't bother with a sperm suit, it was too hot for that, just gloves.
Safety tips- It gets damn hot in a roof space, especially if there is no sarking under tin/tiles, drink heaps of water and get out of the roof space for frequent breaks, heat stroke is very serious and can kill you in a roof.
20 years isnt that old so I would think the electrical work in your roof should be up to scratch but you should never really assume that it is so be careful anything that may be live.
Roof insulation made a massive difference to my place in both summer and winter but I had none to start with, how much difference new stuff makes will depend on the quality of the original stuff.