Dipping my toe into corporate bonds - Advice requested

I'm running out of places to generate a decent return on my savings, and the outlook for me managing to divest some of it by moving up the property ladder is receding. I am therefore thinking it's time to look at using my S&S ISA allowance.

There is a retail bond that has caught my eye. It is the Alpha Plus 7 year 5.75% Secured Bonds. I am tempted because it is secured on property in London. Yes it is schools, so will count as being secured on commercial property, but for me, being secured at all means it compares favourably to any unsecured retail bond I have seen recently.

My only concern is that the proceeds are to be used to repay shareholder debt, and I can't work out from the accounts how much was paid in dividends last year to see if the coupons will be more or less than this, although the operating profit of £4m last year (on turnover of £58m) should provide a decent interest cover regardless.

I was only thinking of sticking about £2-3k in there, and getting the rest of my allowance into some bond fund and some equity fund by the year end, topping them up over next year also (although not UK as there's where my pension fund is still sitting by default). So as I say, just dipping my toes in. However, I had been planning on opening an S&S ISA through Cavendish, as it was recommended on here and my dad has managed to use it to open an ISA investing in an HSBC Tracker fund quite simply and cheaply. But the advert says the Alpha Plus bond is only available through Collins Stewart, iii, Peel hunt LLP, RM Capital Markets, Selftrade, Shore Capital or Smith and Williamson (or Barclays?). Does anyone know which of these I should use that would be cheapest, and wouldn't conflict with my other plans on what to do with my S&S ISA allowance this year and next (or make it more expensive)?

Thanks for your help!