Your cash is comfortable. The risk is one client.
There is £148,400 in the account. £56,500 of that is VAT and corporation tax, so £91,900 is actually yours, against the floor of £60,000 you set. The VAT payment of £21,900 is due in 18 days and the money to cover it is already in.
Marsden Group is now £62,000 of what you are owed, 48% of the total. Two quarters ago it was under a fifth. Nobody decided that; it happened as their invoices grew and everyone else's did not.
Over the same months Marsden stretched from 26 days to pay to 41, and their last accounts reached Companies House 34 days late. More of your money with one client, arriving later, from a company late with its own filings: that is the combination worth watching.
If I were you I would not chase Marsden. I would spend the next quarter making sure the second and third largest clients grow faster than they do, and hold any hire until Marsden is below a third.