I take it you mean the long top across the length of the dash?. You don't need to take the windscreen out at all.
You need to remove the following:
Remove the two padded A-post interior covers. This are fixed in place with simple crosshead self tappers, two per cover.
Under the shelf there are a series of bolts fixed on the underneath of this long top these are bolted to U-shaped brackets in the dash so loosen these nuts. From my recollection, there are four located across the underneath of the long top.
Next, there are two fixing bolts for the centre clock, these bolts will need to be completely removed to remove the clock from the long top, place the clock on the dash shelf.
With the A-post covers off and the nuts loosened/removed, the shelf should slide out with a gentle pull. Take care with the far edges of this long top, these can become quite fragile with age and will need to be clear of any other trim.
I've removed and refitted mine several times to get round the back of the speedo and have also replaced it using this procedure.
Lastly, beware that Series 1 and 2 dashboard tops are not the same, the Series 1 is much lower profile and flatter. A uncracked, unmarked one is becoming quite hard to find. I got mine from Colin at Kingsbury Garage for a very reasonable price, a breakers would be a bit cheaper admittedly.
Weather permitting, I'm taking a visit to my local one to maybe get a few interior bits myself, if I see one, I'll grab it. They're pretty reasonable and as it's not your everyday car, they usually take pity on classic car owners and don't charge the earth.