What ticket do you have to cover the first part of your journey?
None. Will use Oystercard.
OK, you are still covered in the event of delays,
providing you allow the minimum interchange time.
However officially, as you are commencing your journey using Oyster PAYG, you will need to queue at Paddington to get your ticket "endorsed".
If you were using a paper ticket (e.g. a Travelcard), you could simply board the next train.
I don't agree with that policy, but that's what it is, unfortunately.
What was your planned itinerary?
Leave Upminster about 8.15 travelling via Fenchurch Street
9.30 Paddington - Bristol
Sorry that is
not a valid itinerary, so you are not covered in the event of delays. The last train from Upminster to form a booked connection the 0802.
Don't want to go much earlier. 10.00 from Paddington would be just OK but any later and I'd miss event I'm going to.
As the itinerary was never a valid one, and as the reasons are out of control of the Train Companies, I do
not believe you are
entitled to a refund.
However discretion
may be shown, you can ask.
If you are happy to give it a go, you need to be at Upminster in time to make the 0802. Providing you do that, you are covered, this would hopefully give you 63 minutes to cross London for your booked train, or 93 minutes until the 10:00 train, which should be enough time. I'll leave it to others to advise on possible alternative routes to get from Fenchurch St to Paddington.