An agency for which Tesco appear to be their main client.
I would imagine the £13 is for sunday/BH rate including the extra pound an hour for umbrella/SE.
Weekday rates of around £9 sounds about right, but may also include the extra £1 per hour for umrella. Best ask for the PAYE rates and even if they do PAYE come to that! 
As for the “Will include some weekends” implies it is a full time post, but the ad having -£13ph means “will include some weekends” needs to be stated of course. How can an agency bod be told when they are and are not working otherwise? It’ll include the sundays/BH if you are to ever make the £13ph quoted.
I want to work Sundays, BH, and never on a saturday. Friday night is also right out. unless the last day of a 5 day week.
The counting of the pay shall be into double digits for one midium rotation of the clock’s large hand, neither shall the notice given for shifts be less than 24 hours. I’m registered already with several agencies like ADN who apparently won’t give me sundays & BH work unless I do some monday-friday £8.33ph ■■■■■ in the same week. No thanks. That’s not what I (obviously wasted my time) signing up for.
Guess this “Taskforce” is not for me. 