The original Able Staffing went into liquidation in 2020:

https://find-and-update.company-information.service.gov.uk/company/11821456/insolvency

According to the terms of business on their website it has been replaced by three separate companies:

https://ablestaffing.co.uk/terms-of-business/

These seem to be run by the same directors as the original and/or an apparent relative:

https://find-and-update.company-information.service.gov.uk/company/12376612/officers

https://find-and-update.company-information.service.gov.uk/company/12377053/officers

https://find-and-update.company-information.service.gov.uk/company/12383859/officers

The domain they use for their website has been registered for many years and, if you visit their business address on Google Maps, you'll see their offices on the first floor above the Brazilian Steakhouse.

https://www.google.co.uk/maps/place/40-44+The+Broadway,+London+SW19+1RQ

So I think they're a real company. However, £70 is at the top end of what you should be paying for an enhanced DBS check:

https://support.indeed.com/hc/en-gb/articles/360057189032-Disclosure-and-Barring-Service-DBS-Fee-Scams

Are you sure you're talking directly to the agency? Did you contact them through the phone number or email address on their website? If you contacted them via email, have you checked the email headers of the response to confirm it is from their domain? If you are at all unsure, call the phone number on their website and ask to speak to the person dealing with your application.