Roast Beef:How long to cook a 1.75 bone-less sirloin joint to medium in a fan oven?

[time4me]

Please help,this is for a sunday roast for friends coming tomorrow.I want to impress,they are great cooks,both of them.
Also,if a par-boil the roast potatoes,then how can I get them to roast nicely? Do I take the joint out and turn the heat up?
By the way I reckon it is 1 and a half hours.
We dont want to spoil it because we are on a small income and my DH brought this back from Tescos this morning-it cost £25. !!!!!
Thank you very much in anticipation

dont trust tescos timings that is for sure.

last time i checked it was quoting time per kg as it should have been per pound...

Thanks pops

i have the timings somewhere if someone does not help you out soon

DH is on the internet trying to sort it but some reassurance would be nice.Thanks Pops,dont go to too much trouble,I was hoping someone would know it from the top of their head,please dont search for timings

Pops,it went perfectly using a tesco meat thermometer and delia`s method off the internet.Our friends loved the joint and we got lots of compliments from them and they are both foodies.
Thank you for replying,you were the only one and you sound a lovely person