i was braking fairly hard from quite a high speed today and the car was barly slowing down and the pedal was hard i felt if i pushed it any more id lock the brakes

Had you already been braking a lot just before needing them again? Brakes got too hot maybe and faded? If so then you'd not be able to lock the wheels anyway, it feels like brake failure (no more braking to be had, not good!!). If not then check pads and shoes to make sure they are not worn out or got oil/brake fluid contamination on them

When was the last time the fluid was changed?

macplaxton wrote:When was the last time the fluid was changed?

Agreed. Very much so.

How much of the system has been changed in your ownership? What model pads are you using?

Correctly serviced you never need a bootful to operate, even with overspecc'd tyres and an F7R in the lanes.

i ckecked all the pads and discs but i havent even looked at the brake fluid, think ill change it. whats the best fluid i can use?

5.1 should be fine with everything you have, but tbh I've not bothered after the first time I tried. I still needed to change it after 6months, so I just do a 6 monthly change of dot 4.

The boiling point of 5.1 is higher than 4, but it still absorbs water over time, which lowers the boiling point.

Recommended change interval is 18 or 24 months of normal use. Reduce this to a shorter period if under extreme conditions.

Might be like when I bought mine. Fluid hadn't been changed for ages it seemed. Came out black. Full fluid flush solved a lot of problems

Could be any number of problems. If the car is not slowing down much the wheels are not likely to lock.

Pedal travels further to floor after alot of braking= brake fade -> fluid problems

Other problems:
sticking front calliper piston or slider pins
Rear brake adjuster seized/not operating
Blown out flexi hoses
Reservoir rubber plugs blocking the fluid flow into the master cylinder
Poor quality pads
Blockage in pipe

If you have time, cars of this age it is worth a full overhaul. Strip down the callipers, clean, de-rust and paint new seals, check sliders and holes, new hoses. New cylinders and general clean up, new rear hose.

Sounds like the servo?? Without it, stopping is 'ard or non-existant. I would check the vacuum line to this as the ends of the rubber pipe can perish.