Hi, you can use s b k stump killer,just follow the instructions, I have found petrol mixed with old engine oil and pai ted on to thecut surfaces, to be quite effective in killing the stumps, but the rotting process with either method is very slow, if you can get access with a stump grinder, you'll find it's much faster to rot down the remaining roots, once they're below soil level, Derek.

Drill holes down into the stump with minimum 10mm drill bit.

Fill holes daily with neat roundup and/or SBK (Systemic brushwood killer) mixed with diesel 50/50.

Alternatively, I had a tree cut down to about soil level, had it painted with some stuff to kill roots and prevent it from re-growing. Applying the philosophy: "It won't bother me when I don't see it", I put a bit pot on it with a beautiful plant.

I would bite the bullet and hire or get someone in with a stump grinder. Yes, it might be expensive but the job will be over and you can get on with new plantings instead of waiting ages for the stumps to rot away.