Is there a need to do that in order to get 8:21 or 8:51 train? Thanks.

There are no direct trains Paddington to Windsor - always change at Slough.

You can reserve a seat on Paddington to Slough trains -optional and do so when buying tickets - no discounts for buying early or minimal I would think-just buy at station - this is rush hour but at that time most folks will be going into London not out - I'd just pop to station and buy - rare as a sunny day in London to see any trains like those full and then you could still board. Just get to Paddington early and board early to get seats- which I would think there will be plenty of anyway going out of London.

For variety you may want to return to London on trains to Waterloo Station -right by the Eye Over London -this is a leafy suburban route vs the boring industrial mainline from Paddington.

I would not worry about it at all. But if sure and want to book www.nationalrail.co.uk.

Thanks Pal. Appreciate it. By the way you don't have Fisher to kick around any more...but we still have Dutcher.

did not hear Fish had sunk!

You can take a direct train from Waterloo w/ no changes, or a train from Paddington w/ a change at Slough

Whichever you take, it will cost more if you leave before 9:30

"You can reserve a seat on Paddington to Slough trains "

On most you can't.

The answer to your question is a simple "No."