Landscaping quote

We're looking to get our garden re-landscaped - we already have a concept design from a designer we've engaged and we've finalised most of the details and she'll shortly be working on the final presentation and planting plan.

We've now had three quotes to get the work done. It's a complete clearance and start from scratch job, mostly. The approximate garden area is 8m wide 20m deep. The brief I've given, along with the concept plan is, in summary:

* Clear all existing hard and soft landscaping (mostly the old patio, paths, shingle areas, a single raised bed and plants). Cut away concrete from area for new garden office, mark out, level and prepare the site.
* Rectify an issue with the boundary fence - we recently had it replaced but it's ended up 3-4 inches higher than the ground level in places - remove existing gravel boards/panels, dig down and sink in an extra 6" gravel board to close the gap.
* Dig trenches for supply of power and data cabling to garden office building (supplied and installed by a third-party).
* Fit 4 and a half new fencing bays to secure rear boundary of the garden as per design.
* Construction of new patio and paths - approx. 44m2 - using Stonemarket Lucent Copper vitrified paving - excavate, prepare sub-base with 100mm compact type 1 MOT, full wet mortar bed, slurry adhesive to back of flags, use Stonemarket's own wet brush in resin pointing.
* Dig and prepare hole for new 8ft sunken trampoline, install sunken trampoline according to manufacturer's instructions (supplied by us).
* Soft landscaping - prepare ground, supply and lay topsoil, new hard wearing turf (approx. 40 m2). Prepare all new flower beds and borders for planting - dig over, incorporate organic matter/soil improver. Planting and plant supply to be undertaken by our garden designer.
* Dispose of all waste.

All in all, it's mostly a straightforward job, with the most expensive part (apart from the garden office, which we have budgeted for separately) being the hard landscaping - the vitrified paving is about £65-70/m2. Our budget was around £15k and we've already allowed for £1k on planting and another £1k for the various bits we need to buy ourselves (the trampoline, a hazel play den for our children, some raised beds and vegetable soil mix for the vegetable garden).

The three quotes we've had are, including VAT: £17.5k, £15.5k and £9k. We've ruled out the first - their quote was quite detailed and broken down and they have reduced the costs a bit from their initial quote but it still blows our budget.

The middle quote is over budget but we can just about stretch, and our designer has worked with these landscapers on several projects before.

The cheapest quote seems too cheap to me - I've asked for clarification on whether that includes the cost of the flags as they come to nearly £3k alone. If it's without, then that's still about £12k total. If its including the flags, then it just seems way off.

But I could be wrong, maybe the first two are overpriced and the cheapest quote (assuming cost of flags is on top) is about right? Only the most expensive quote had a full break down and their quote included £9k on the hard landscaping and trampoline install alone.

Any ideas? The saving of the cheapest quote is nothing to be sniffed at and their work looks good from what I have seen. They don't seem like cowboys.