If it is a long completion date couples things you need to aware of:
1. You need to aware of the property valuation, for example you brought the flat 6 months again. After six month is your completion date, you need to check the market value of the flat. If the price is lower then your mortgage amount will be lower.
2. If you have apply for an mortgage, usually the bank will re-do the valuation for you for the current price.
3. Also, you need to re-submit all the income documents and need to re-do the credit checking.
This is very risky in the long completion date as the future of the market is un-known.