Thanks everyone: I discovered what's wrong with that our awning and it is now fixed. There were in fact 4 things wrong with it and I am posting this in case it helps anyone else whose awning has collapsed and broken in the rain.
1) the front awning support bracket was bent. I could have tried to straighten it but I couldn't work out how or where it was bent and I was also concerned that the bent one would be inherently weak as it takes all the strain. The brackets are only available directly from VW or indirectly e.g. via the club shop or TPS. They are about £70 each. The VW part numbers are:
Front (black) 7H7 877 238 B9P 7 (silver) 7H7 877 238 C8Q 8
Rear (black) 7H7 877 238 D9P 7 (silver) 7H7 877 238 B8Q 8
This substantially improved the sag but it still wasn't right.
2) I then decided to remove the spring arm and discovered that the upper pivot bolt on the inner end of the arm was bent back and it was actually the cast end piece that was twisted not the bolt. No parts are available for the spring arms of Thule 5102 awnings and you have to buy them complete for £187 each from the club shop. Before I did this, I decided to peruse the Thule awning spares list and saw that a similar part was offered as a spare for the bigger Thule 4900/6200 awnings, so I emailed Hill View Awnings to ask them if it would fit a 5102. They very kindly got one out of stock to compare with the one on a 5102 spring arm and emailed back to say it was different but it looked like it would fit. So, I took a punt and bought it on line from them.
As you'll see from the photo below, it is chunkier but it was dimensionally identical and fitted perfectly. It came with fitting instructions and you have to drill the old one where I have circled in order to be able to remove the hinge pin. Then you have to punch the old in a dimple that's provided.
The Thule part number is 1500602811 and the same part will fit either spring arm: £30.85 plus p&p
This again substantially improved the sag but not enough to be able to wind the awning in without it catching on the open sliding door.
3) The lower pivot bolt of part number 1500602811 rotates in an extruded aluminum bracket fixed to the awning housing. The upper bolt moves transverse itself in an oval-section slot in the bracket according to the angle of the spring arm. On further inspection, I could see that the bracket had some bruising and that the oval slot looked wider at one end meaning that the arm could sag more than it was supposed to (see pic). Again, this part comes as complete assembly with the spring arm and isn't available separately. However, before I resigned myself to a new arm, I again perused the Thule awning spares list and right at the end, just as I was losing heart, I saw a mounting bracket used for fitting a G2 side blocker panel to a 5102 awning that looked the same.
Again I took a punt and luckily the bracket is identical to the one used for the spring arm.
The Thule part number is 1500603376 and the same part will fit either spring arm. £21.80 plus p&p
4) This fully cured the sag but it still just caught the sliding door. I then noticed that the front rail was tiny bit distorted near the outer end of the arm, so I bent it back with a plumbers wrench and I'm pleased to say it now works perfectly and the damage cost me about £132 all in as opposed to the £250 it would have cost if I'd bought a new arm.
Lesson learned though: never leave an awning in the rain unless there's a huge side-to-side angle because a small angle may not be enough if there's a biblical downpour!
New front bracket fitted: note the long rubber drip seal pressed into the awning rail behind.
Note the top bolt only has a Nylon washer on its outer end. I thought I'd lost it at first then noticed the other arm only had 3 Nylon washers. Don't tighten them too much as they need to move.