Moving house used to be so easy.

Offers on our house and the one we want to buy have been accepted, yo, then we got a rather large packet of legal documents from our solicitors. I wish there had been a covering letter to say they'd sent duplicates to let us know there was one we had to fill in for them and another for our records. Cue Goosey on her knees trying to sort them all out with various accompanying words like "Hang on, I've just seen that doc which I filed on the right-hand side of my chair. Don't tell me I've got two lots of docs, der! Having got to grips with that possibility I continued to sort them all out into docs that were just info only and those that had to be filled in. All was going well until I found two docs with the same cover page but their contents differed. My OH and I managed as best we could but we've got an appointment with our solicitors this afternoon so they can sort it all out for us!

We're going through the same right now, and yes it's a pain!
Mercifully we have a decent solicitor and the estate agents involved are great. Just want to get it done ASAP though as I want to show off the blank canvas garden we'll have

House move last year after 42 years was the most traumatic thing ever.  Chain of 3,   only sorted when our buyer told his buyer that it they did not complete on the day they had suggested he would put his house back on the market and refuse to sell it to them. 

I think the buyers at the bottom  were hoping he would drop the price in order to speed things up. 

Ok MB. Whoever is buying a home from your prospective buyer may need a survey as part of a condition of their mortgage or whatever. Once their survey is done and all goes well, the people who want to buy your Mum's place can then proceed (unless their survey goes doo-dahs up) to fill in all the documentation along with evidence that they have enough funds to purchase your mum's home.
Do this for me.
1) You shouldn't have been advised to sign a contract before buyer number one had a survey done on buyer number two's house in case it needed a lot of remedial work because it may have resulted in a bit of haggling for a lower price.
2) Don't pay any money to your solicitor in case it all falls through because you'll lose it all.
3) Don't sign anything else until you have legal documentary proof from buyer number two to state they are still in a financial position to buy your mum's house.

I need to correct some legal inaccuracies here.

1) There is no risk in signing your part of the contract early.  It is not a valid document until everyone is happy with the chain, and no further changes need to be made, and all the solicitors at that time complete the necessary outstanding pieces of the contracts - at which time they become legally binding documents.  Until that time, they can be updated and re-signed without you incurring any legal liability.

2) It is either misleading, or categorically wrong, to say that you will lose any money you pay to your solicitor before exchange of contracts - depending on what money you are referring to
- If you are having searches etc. done for your purchase, then you have to pay that money up front in order for your solicitor to carry out those searches etc.  Entirely up to you whether you pause your purchase pending the buyers at the bottom of the chain getting a survey done, but bear in mind that if you do stop making any progress on your purchase then others may become worried that you are not committed to moving, and thus pull out of buying your place. 
- In relation to money for the deposit for your purchase, this is 100% safe and there is absolutely no risk of you losing it.  To say otherwise is scaremongering.  It is held by your solicitor on your behalf in protected accounts.  If the purchase does not go ahead, you will get it all back.  Of course, you will still have to pay your solicitors' fees unless you have a No Fee deal.

3) I am not sure what other documents you may still have to sign.  However, if you're not sure then ask your solicitor.  As I already said, you are not committing yourself to anything by signing your half of documentation - it does not become valid unless and until the chain goes ahead.  It is standard good practice for you to be asked by your solicitor to sign various documents in advance so that you are in a position to proceed immediately - things can happen very quickly at the end.

Sorry to word this so strongly but it's important to ask your solicitor if you have any worries or concerns, rather than rely on well-meaning advice from friends off the internet.

Glad it was helpful in part Mrs B 

Yorkie, please accept my apologies for any inaccuracies I posted. The money I was referring to is an up-front fee for the work your solicitor has done to date. According to my mortgage advisor, if the buyers for your house or the vendors for your new house pull out for what ever reason you will lose that money.

No worries - you're completely right about the disbursements which you have to pay before the solicitor does the work