Is this stated in my contract with talktalk or is guidelines from ombudsman?
AFAIK BT fault repairs are done on a best effort basis.
You should expect some sort of action within a couple of working days.
It depends on the 'Care level' ISP's have with Openreach. Most ISP's use level 2 but a couple have a level 1 care level. More info from Openreach:
Service benefits Maintenance Level target fix times
Level 1 Clear by 23.59 day after next, Monday to Friday, excluding Public and Bank Holidays. For example, report Tuesday, clear Thursday.
Level 2 Clear by 23.59 next day, Monday to Saturday, excluding Public and Bank Holidays. For example, report Tuesday, clear Wednesday.
Level 3 Report 13.00, clear by 23.59 same day. Report after 13.00 clear by 12.59 next day, seven days a week, including Public and Bank Holiday.
Level 4 Clear within 6 hours, any time of day, any day of the year.
Needless to say higher care levels cost substantially more and I'm unaware of any domestic ISP that goes beyond level 2. Business ISP's of course could choose 3 or 4 depending how critical the service is and how much the company is willing to pay. The price difference between Level 1 and level 2? £2 a year per customer last I heard. As you can see as well, level 1 only includes Monday to Friday appointments and 'normal' working hours, 8am-1pm or 1pm-6pm. It has caused issues for teachers etc. in the past for us!
It should also be noted that the 'clear' is an Engineer attending the fault, not guaranteed resolution. If dig work is required that's not on a path or road then AFAIK it's 24 hour notice to the council. Path 48 hours, Road with traffic management/lights can be a week, road closure? Up to 28 days!
I had one customer who lived about a dozen yards from the A1 North of Newcastle who's fault was located under the road. The council gave permission after three weeks for the dig work. On a Sunday morning, starting 7am! He got woken up!