Most leases are straightforward and don't need a solicitors opinion. Just make sure that all questions are answered in the lease: amt of rent, rent review period, length of lease, what the landlord provides/is covered by the service charge, maintenence etc, what your responsibilites are, alarm system, fire escape etc. Don't worry ttoo much about the length of the lease - it's a renters market at the moment (though that might depend on where your office is) but do negotiate anything you want changed - rent, lease length etc.
Don't forget to ask about service charge and rates- these can mount up to quiite a bit.
the reason most leases are less than 5 years is that at five years you accrue rights that residential leasees can only dream of.