I own one flat in a block of six. We have had to get some communal

Thank you for your question.
The title deeds to the property may provide for this type of situation, but, typically, the five owners who have paid are entitled to pursue the sixth for his or her share.
You would pursue this as a common debt by writing it to the owner in the first instance. If payment was not forthcoming you would be able to pursue the debt in the Small Claims Court in the usual way.
The other five owners can also record a notice in the Registers of Scotland which would prevent the sixth owner from selling or granting any security over their property without paying off the sums due. You can read more about this in sec 13 of the Tenements (Scotland) Act 2004 as follow:
"13 Notice of potential liability for costs: further provision
(1) A notice of potential liability for costs
(a) may be registered in relation to a flat only on the application of
(i) the owner of the flat;
(ii) the owner of any other flat in the same tenement; or
(iii) any manager (within the meaning of the Tenement Management Scheme) of
the tenement; and
(b) shall not be registered unless it is signed by or on behalf of the applicant.
(2) A notice of potential liability for costs may be registered
(a) in relation to more than one flat in respect of the same maintenance or work; and
(b) in relation to any one flat, in respect of different maintenance or work.
(3) A notice of potential liability for costs expires at the end of the period of 3 years beginning with
the date of its registration, unless the notice is renewed by being registered again before the end of
that period.
(4) This section applies to a renewed notice of potential liability for costs as it applies to any other
such notice.
(5) The Keeper of the Registers of Scotland shall not be required to investigate or determine whether
the information contained in any notice of potential liability for costs submitted for registration is
accurate.
(6) The Scottish Ministers may by order amend schedule 2 to this Act"
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